Core: Release the read lock while creating connections in refresh_connections

[barshaul]

Lock Management Improvement:

In the previous implementation, the read lock (inner.conn_lock.read()) was held throughout the entire connection refresh process (for all connections sent to refresh), including while attempting to establish connections (via get_or_create_conn). If connections were slow or timed out, the lock was held for an extended duration, blocking other tasks requiring a write lock.

The new implementation releases the read lock before making connection attempts. If the connection is successfully established, the read lock is reacquired to update the connection container. This approach ensures that other operations needing the lock (e.g., write operations) can proceed while connections are being established.

This commit was already approved on our redis-rs fork but wasn't merged into glide, see former PR amazon-contributing/redis-rs#191.