Use ReentrantReadWriteLock in AbstractConnPool

AbstractConnPool is currently using a ReentrantLock which has to lock for read and write operations. Switching to ReentrantReadWriteLock[1] and read locks for the stats/getters methods would reduce the possibility for instrumentation[1] to cause contention. Another option would be StampedLock[3] if the compile target is 1.8.

[1] https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html
[2] https://github.com/dropwizard/metrics/blob/v3.2.2/metrics-httpclient/src/main/java/com/codahale/metrics/httpclient/InstrumentedHttpClientConnectionManager.java#L63-L95
[3] https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/StampedLock.html