Major Description
This issue was observed after upgrading to httpclient v4.5.2 and httpcore v4.4.4.
When configuring the PoolingHttpClientConnectionManager, I have set validateAfterInactivity to 1ms as this is the only way of maintaining the old behaviour, i.e. checking every connection if is stale before using it.
I have observed a performance degradation under high load when the httpclient is shared between multiple threads. This httpclient is used only for one route.
After taking a thread dump, one thing that got my attention was several threads waiting for same ReentrantLock instance while trying to AbstractConnPool#getPoolEntryBlocking/AbstractConnPool#release. The ReentrantLock instance in question was owned by another thread performing CPool#validate.
It seems to me that performing this stale check inside the region protected by this lock is unnecessary and also induces a big performance hit when using the httpclient from multiple threads.
I've atached a thread dump of a test application that reproduces this behaviour. ReentrantLock in question in this thread dump is 0x0000000706e8b9a8 and is owned by Thread-4.
