MB-7635 Increase synchronization timeout to ns_config_rep_merger.

ns_config_rep:synchronize_remote function uses timeout of 30
seconds. But internally corresponding handle_call clause used default
gen_server timeout (which is 5 seconds) to synchronize with merger
process. This made outer timeout basically useless.

This is exactly what I see in MB-7535 log. For some non-obvious reason
one of the nodes fails to synchronize with merger process within 5
seconds which crashes ns_config_rep process on that node.

Unfortunately I don't know what the merger process was occupied with
for those 5 seconds. Some evidence suggests (e.g. lost ticks messages)
that it may be another manifestation of vm timeouts. Or it may be
something else. Anyway, this change should make the issue less visible.

Change-Id: I867bbe5ba7c3be734a64593f9198be4b5edbcdac
Reviewed-on: http://review.couchbase.org/24360
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>