At most 2 connections can be in the pending state per connection pool.
The pending state covers all the work required to setup a new connection
including TCP, TLS, and MongoDB authentication. For example, if two
threads are currently creating connections, a third thread will wait for
either an existing connection to be checked back into the pool or for
one of the two threads to finish creating a connection.
The change reduces the likelihood of connection storms and improves the
driver's ability to reuse existing connections.
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