MB-8190: fixed race in janitor_agent:handle_call_via_servant

handle_call_via_servant was introduced to avoid blocking janitor_agent
for requests that don't need to be serialized with rest of
janitor_agent activity.

And in original implementation I was trying to be simple and short yet
tried to avoid some additional burden on GC.

So handle_call_via_servant works by spawning servant for doing actual
call and by replying it's pid and reply tag. Then original caller
would await servant's death and receive servant's reply.

By trying to avoid additional burden on GC I made original code reuse
$gen_call tag for actual reply from servant. But that made it
vulnerable to a race where servant would reply before "original" reply
with servant's pid. Which, predictably, caused badmatch.

The fix is to cleanly separate those two replies by having servant's
reply using new unique tag.

Change-Id: I39d790c08175a680c6dfbfd79a78c43cb1102baa
Reviewed-on: http://review.couchbase.org/26047
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>