OPENDJ-858 (CR-1651) Add stats tracking to HTTP client connections 

Added support for monitoring statistics of the HTTP connection handler:
It monitors: total number of requests, and number of delete, get, patch, post and put requests.
It also monitors the internal LDAP operations to display how the server is performing.


HTTPConnectionHandler.java, HTTPClientConnection.java, CollectClientConnectionsFilter.java, SdkConnectionAdapter.java:
Added support for statistics.

config.ldif, HTTPConnectionHandlerConfiguration.xml, HTTPConnectionHandlerCfgDefn.properties:
Added "keep-stats" property.

02-config.ldif:
Added attributes "ds-mon-http-*" for monitoring HTTP statistics.

LDAPStatistics.java:
Changed getMonitorData() return type to List.
Made createAttribute protected.

HTTPStatsProbe.java, HTTPStatistics.java: ADDED