[4.0.2] CBG-5007: Split ServerContext mutexes

[bbrks]

CBG-5007

Unclean cherry-pick of #7889 for 4.0.2

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/177/

[Copilot]

Pull request overview

This PR splits the single ServerContext.lock mutex into two separate locks (_databasesLock and _httpServersLock) to reduce mutex contention and improve concurrency. The change renames database-related maps to have underscore prefixes (_databases, _dbConfigs, _dbRegistry, _collectionRegistry) and updates all references throughout the codebase to use the appropriate lock for each data structure.

Key Changes

• Introduced separate locks: _databasesLock for database state and _httpServersLock for HTTP server management
• Renamed internal maps with underscore prefixes to indicate they are protected by _databasesLock
• Refactored Close() method to stop HTTP servers first, then clean up databases and other resources in a logical order

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rest/server_context.go	Core changes: split mutex into _databasesLock and _httpServersLock, renamed maps with underscore prefixes, refactored Close() method
rest/utilities_testing.go	Updated lock usage to _databasesLock, added updatePersistedConfig helper for testing
rest/handler_config_database.go	Updated to use _databasesLock instead of lock
rest/admin_api.go	Updated all database map accesses to use _databasesLock
rest/config.go	Updated all database configuration operations to use _databasesLock
rest/serverless_test.go	Updated test assertions to use renamed underscore-prefixed map fields
rest/config_test.go	Updated test assertions to use renamed _dbConfigs field
rest/api_test.go	Updated tests to use new updatePersistedConfig helper instead of direct map access

[Copilot]

The function creates a map with the same size as sc._databases and then copies all entries. This is redundant - maps.Copy can copy into an empty map without pre-sizing. Consider removing the size parameter from make() or using a simple loop if pre-sizing is intentional for performance.

Suggested change

	databases := make(map[string]*db.DatabaseContext, len(sc._databases))
	databases := make(map[string]*db.DatabaseContext)