WIP Add LedgerStateCache

[ThomasBrady]

Adds LedgerStateCache to store all Soroban entries:

• Populates (LedgerStateCache::addEntry) the cache in PopulateLedgerCacheWork (executed in LedgerManagerImpl::loadLastKnownLedger).
• Updates the cache (LedgerStateCache:::addEntries) each ledger from LedgerManagerImpl::transferLedgerEntriesToBucketList.
• Reads the cache (LedgerStateCache::readEntry) in LedgerTxnRoot::Impl:::getNewestVersion.

Notes:

• LedgerStateCache lives in an optional shared pointer and is created at startup, originally in the LedgerManagerImpl
• LedgerTxnRoot takes an std::optional<LedgerStateCache> in its constructor, and PopulateLedgerCacheWork access it with LedgerManager::getLedgerStateCache.
• addEntry and addEntries are private and only accessible from friend classes LedgerManagerImpl and PopulateLedgerCacheWork
• addEntry and addEntries acquire a unique lock on the cache
• Currently, the LedgerStateCache only supports contract entries. To access them in the index, I've added Bucket / Index::getContractEntryRange.
• The LedgerStateCache is enabled via Config::IN_MEMORY_SOROBAN_STATE_FOR_TESTING (default true).

Description

Resolves https://github.com/stellar/stellar-core/issues/4556

Checklist

• [ ] Reviewed the contributing document
• [ ] Rebased on top of master (no merge commits)
• [ ] Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• [ ] Compiles
• [ ] Ran all tests
• [ ] If change impacts performance, include supporting evidence per the performance document