set forestdb buffer cache size to 1mb

Otherwise it's set to 128 megs and forestdb somehow allocates all that
memory immediately on database open. Which causes even empty gometa
instance to eat 140+ megs of ram on my box for no reason at all.

Change-Id: I30f0f3a8ac3e784765024ae0e44185721b5aa299
Reviewed-on: http://review.couchbase.org/42336
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>