Move ns_mnesia:ensure_table into the gen_server
Move ns_mnesia:ensure_table into the gen_server

We were having crashes from trying to create tables while shutting
down.

Change-Id: I5dc2f951abc6036558d040890e821ed34a838594
Reviewed-on: http://review.northscale.com/3184
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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