Fix race on bucket random evicion cache

[SirTyson]

Description

Resolves https://github.com/stellar/stellar-core-internal/issues/492

Originally I was going to create a thread-safe version of the random eviction cache do reduce contention, but until we observe a perf bottleneck related to this cache, just changing the lock to exclusive is the simplest fix.

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

[Copilot]

Pull request overview

Fixes a concurrency issue in the LiveBucketIndex random-eviction cache by ensuring cache lookups take a write/exclusive lock when they mutate cache-internal state (access counters / generation / last-access).

Changes:

• Switch cache lookup locking in LiveBucketIndex::getCachedEntry from shared to exclusive to prevent concurrent mutation races.