CBG-5472: Make the stats logger independent of the database lock

[bbrks]

CBG-5472

The stats logger could stall, leaving gaps in sg_stats.log, whenever a database config update held a lock the stats path also needed. Two independent locks were involved:

• _databasesLock: updateCalculatedStats took RLock, but a config update holds the write lock across an infinite index-readiness wait, blocking the reader for the whole (unbounded) window.
• base.SgwStats.dbStatsMapMutex: String() held it for the entire marshal, and NewDBStats/ClearDBStats held it for all Prometheus (un)registration under _databasesLock.

Decouple the reader from both:

• ServerContext now keeps a lock-free atomic snapshot of the database contexts, refreshed under _databasesLock at each _databases mutation. updateCalculatedStats reads the snapshot, never the lock.
• dbStatsMapMutex is now held only for the map operation: String() marshals a shallow copy, and NewDBStats/ClearDBStats do their Prometheus work outside the lock. The unregister* calls only touch Prometheus (not the DbStats maps), so this stays race-free.

The stats ticker now keeps emitting (stale-but-present) while databases are added, removed or reloaded.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api
• Self-review @bbrks

Dependencies (if applicable)

• CBG-3658: Prevent non-finite CPU stat from corrupting stats serialization #8430

Integration Tests

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/0000/