Add double lock check in connection pool acquire

From PR:
"During load testing I noticed several time intervals where client stops to send requests to the server, after adding additional logs was discovered that if client activity increases dramatically and you don't have enough connections in the pool all current requests will wait in lock and will try to open new connection one by one. Opening new socket connection takes time and almost for sure that some connections will be released while operation waits in lock. The fix is simple check if connection was released before opening new one."