#### Problem
Accounts db currently use AppendVec::new_from_file() directly
to create a new AcountsFile instance from an existing file.
However, this method should be abstracted out to AccountsFile
so that an existing file can be opened correctly using the right format.
#### Summary of Changes
Add new_from_file() API to AccountsFile which will open an existing
file based on its accounts file format.
Currently, it only supports AppendVec.
* Add accounts hard-link files into the bank snapshot directory
* Small adjustments and fixes.
* Address some of the review issues
* Fix compilation issues
* Change the latest slot snapshot storage from VecDeque to Option
* IoWithSourceAndFile and expanded comments on accounts
* last_slot_snapshot_storages in return value
* Update comments following the review input
* rename dir_accounts_hard_links to hard_link_path
* Add dir_full_state flag for add_bank_snapshot
* Let appendvec files hardlinking work with multiple accounts paths across multiple partitions
* Fixes for rebasing
* fix tests which generates account_path without adding run/
* rebasing fixes
* fix account path test failures
* fix test test_concurrent_snapshot_packaging
* review comments. renamed the path setup function
* Addressed most of the review comments
* update with more review comments
* handle error from create_accounts_run_and_snapshot_dirs
* fix rebasing duplicate
* minor accounts_dir path cleanup
* minor cleanup, remove commented code
* misc review comments
* build error fix
* Fix test_incremental_snapshot_download_with_crossing_full_snapshot_interval_at_startup
* fix build error on MAX_BANK_SNAPSHOTS_TO_RETAIN
* rebase fix, update hardlink filename
* minor comment spelling fix
* rebasing fixes
* fix rebase issues; with_extension
* comments changes for review
* misc minor review issues
* bank.fill_bank_with_ticks_for_tests
* error handling on appendvec path
* fix use_jit
* minor comments refining
* Remove type AccountStorages
* get_account_path_from_appendvec_path return changed to Option
* removed appendvec_path.to_path_buf in create_accounts_run_and_snapshot_dirs
* add test_get_snapshot_accounts_hardlink_dir
* update last_snapshot_storages comment
* update last_snapshot_storages comment
* symlink map_err
* simplify test_get_snapshot_accounts_hardlink_dir with fake paths
* log last_snapshot_storages at the end of the loop
* Add run parent directory for accounts files
* fix test test_concurrent_snapshot_packaging
* review comments. renamed the path setup function
* Addressed most of the review comments
* remove explict type def for map result
* handle create_accounts_run_and_snapshot_dirs error with expect
* update with more review comments
* minor fixes from review comments
* simplify account_filename option assignment
* handle error from create_accounts_run_and_snapshot_dirs
* use then instead of then_some for lazy evaluation
* Clean up files in the old account_path before trasitioning to the new run path
* try_exist and accounts_dir removing extra
* sync rmdir, is_dir check
* handle the account_path not deletable case
* Add run parent directory for accounts files
* fix test test_concurrent_snapshot_packaging
* review comments. renamed the path setup function
* Addressed most of the review comments
* remove explict type def for map result
* handle create_accounts_run_and_snapshot_dirs error with expect
* update with more review comments
* minor fixes from review comments
* simplify account_filename option assignment
* handle error from create_accounts_run_and_snapshot_dirs
* use then instead of then_some for lazy evaluation
* Add snapshot_storage_lengths_from_fields
* Add SnapshotUnpacker
* Add SnapshotStorageRebuilder
* Rebuild snapshot storage during unpack
* move snapshot unpacker back into snapshot_utils
* use SerializedAppendVecId in storage rebuilder
* create AccountStorageMap type alias
* Wrap storage and next_append_vec_id in a type
* fixed typo
* move use StorageAndNextAppendVecId
* calculate num_rebuilder_threads more concisely
steviez
Brooks
Yueh-Hsuan Chiang
Xiang Zhu
Jeff Washington (jwash)
apfitzge
Brennan Watt