refactored stats collector for easier stats kinds additions
refactored stats collector for easier stats kinds additions

Particulary separate concerns are separated and code that massages
different kinds of stats is separated as well.

Change-Id: I0db8e3f18c8ab8dde4df6d1e8068acf764d191d8
Reviewed-on: http://review.couchbase.org/14753
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

mschoch/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,membase/ns_server,vzasade/ns_server,nimishzynga/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,daverigby/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server,vzasade/ns_server,daverigby/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,vzasade/ns_server,daverigby/ns_server,membase/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,membase/ns_server,vzasade/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,mschoch/ns_server