Fix a connection pool error.

[mikkokar]

Summary

This PR fixes a little mistake in a SimpleConnectionPool that may allow more than allowed connections to be created.

Root Cause

A borrowCount was incorrectly used check if we need to create a new connection or not. But there is a problem:

• A connection establishment is an asynchronous operation, and borrowCount is incremented only after the establishment has completed, and the new connection is handed out to a pool consumer.

• Due to TCP 3 way handshake and a potential SSL handshake there is a considerable time lag between start of the connection establishment and borrowCount being incremented.

• Extra connections can get created within this time window.

Fix

Add an extra counter to track the number of connections under establishment. Only trigger a connection establishment if borrowCount + connectionsUnderEstablishment is less than maximum allowed.