MB-5307: allow init and sync task submits in generic work queue
MB-5307: allow init and sync task submits in generic work queue

This makes wider class of problems solvable with just work queue
instead of full-blown gen_server.

Change-Id: I79e26342fe04473a2434ec6d462849974b13faa0
Reviewed-on: http://review.couchbase.org/17754
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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