* Connection pool in connection cache and handle connection errors
1. The connection not has a pool of connections per address, configurable, default 4
2. The connections per address share a lazy initialized endpoint
3. Handle connection issues better, avoid race conditions
4. Various log improvement for help debug connection issues
* client: Remove static connection cache, plumb it instead
* Add TpuClient::new_with_connection_cache to not break downstream
* Refactor get_connection and RwLock into ConnectionCache
* Fix merge conflicts from new async TpuClient
* Remove `ConnectionCache::set_use_quic`
* Move DEFAULT_TPU_USE_QUIC to client, use ConnectionCache::default()
* Fixup typo
* Add new feature
* Add new TransactionError
* Add framework for checking account state before and after transaction processing
* Fail transactions that leave new rent-paying accounts
* Only check rent-state of writable tx accounts
* Review comments: combine process_result success behavior; log and metrics before feature activation
* Fix tests that assume rent-exempt accounts are okay
* Remove test no longer relevant
* Remove native/sysvar special case
* Move metrics submission to report legacy->legacy rent paying transitions as well
* Move test-validator to own module to reduce core dependencies
* Fix a few TestValidator paths
* Use solana_test_validator crate for solana_test_validator bin
* Move client int tests to separate crate
Co-authored-by: Tyera Eulberg <tyera@solana.com>