#### Problem
In TieredAccountMeta, RENT_EXEMPT_RENT_EPOCH will be used when
its optional field rent_epoch is None. However, for legacy reasons, 0
should be used for zero-lamport accounts.
#### Summary of Changes
Return 0 for TieredAccountMeta::rent_epoch() for zero-lamport accounts.
#### Test Plan
accounts_db::tests::test_clean_zero_lamport_and_dead_slot
#### Problem
While accounts-db might not invoke appends_account twice
for the same AccountsFile, TieredStorage::write_accounts()
itself isn't thread-safe, and it depends on the above accounts-db
assumption.
#### Summary of Changes
This PR makes TieredStorage::write_accounts() thread-safe.
So only the first thread that successfully updates the already_written
flag can proceed and write the input accounts. All subsequent
calls to write_accounts() will be a no-op and return AttemptToUpdateReadOnly
Error.
#### Problem
There're some test functions that have been used in different
mod in TieredStorage. It's better to have one same place for
all tiere-storage related test functions.
#### Summary of Changes
Created test_utils.rs under /tiered_storage and move test-related
functions into it.
#### Test Plan
Existing tests.
#### Problem
While the implementation of hot-storage reader and writer
are mostly done, it is not yet connected to TieredStorage.
#### Summary of Changes
This PR enables hot-storage in TieredStorage::write_accounts().
#### Test Plan
Completes the existing tests in TieredStorage to directly
write and read from a TieredStorage with the hot storage format.
#### Problem
append_accounts() only appends (len - skip) accounts.
However, AppendVec::append_accounts() reserves `len`
instead of `(len - skip)` for its vectors.
#### Summary of Changes
Use (len - skip) as the initial size of the Vectors.
#### Problem
HotStorageReader and TieredStorageReader haven't implemented
accounts() that is required by AcocuntsFile.
#### Summary of Changes
This PR implements accounts() for both HotStorageReader
and TieredStorageReader
#### Test Plan
Extend the existing test to cover accounts().
#### Problem
TieredStorageMeta and TieredStorageReader::get_account API uses
u32 to represent IndexOffset. However, within the TieredStorage scope,
IndexOffset should be used, it is not until working with AccountsFile API
when u32 representation of offset is needed.
#### Summary of Changes
Have TieredStorageMeta and TieredStorageReader to use IndexOffset.
#### Test Plan
Existing unit-tests.
#### Problem
To allow hot-storage to use HotStorageWriter::write_account() to
implement AccountsFile::append_accounts(), it is required to
provide a Vector of StoredAccountInfo to allow AccountsDB to
properly prepare the entry for each account.
#### Summary of Changes
This PR enables HotStorageWriter::write_account() to return
Vec<StoredAccountInfo>.
#### Test Plan
Extend existing tests for HotStorageWriter to verify the correctness
of the returned Vec<StoredAccountInfo>.
Brooks
Yueh-Hsuan Chiang
Brooks
Dmitri Makarov
Pankaj Garg
Jeff Washington (jwash)
HaoranYi