Add statics (Servo counters) to measure the efficiency of the default connection pool manager.

Here is how it is done:

- Created a new connection pool class NamedConnectionPool that extends from ConnPoolByRoute. Added relative counters before/after calling super.
- Created a new class MonitoredConnectionPoolManager, which extends ThreadSafeClientConnManager and uses NamedConnectionPool as the underlying connection pool.
- Updated NFHttpClient to use MonitoredConnectionPoolManager by default for named client.
- JUnit test is provided to make sure counters are incremented as expected.

Also upgraded httpclient version to 4.2.1.