MB-7758: implemented faster misc:dir_size

On my box it appears to be massive improvement.

(n_0@10.17.37.209)9> timer:tc(dir_size, get, [BD]).
{734,3357296}
(n_0@10.17.37.209)10> timer:tc(dir_size, get, [BD]).
{692,3357296}
(n_0@10.17.37.209)11> timer:tc(dir_size, get, [BD]).
{962,3357296}
(n_0@10.17.37.209)12> timer:tc(dir_size, get_slow, [BD]).
{13695,3357296}
(n_0@10.17.37.209)13> timer:tc(dir_size, get_slow, [BD]).
{14616,3357296}
(n_0@10.17.37.209)14> timer:tc(dir_size, get_slow, [BD]).
{13059,3357296}

Unfortunately while go is easy to cross-compile and run almost
anywhere, most of cost is some initialization of go executable. I
think 3 milliseconds is a still a bit on higher side. So we have to
run it as port.

We're sending port netstring of path, and receive LF-terminated json.

Change-Id: I2f153bbd700809b9afe23141b6fe6e8bc958ad83
Reviewed-on: http://review.couchbase.org/24617
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
MB-5307: avoid parsing value that we already know

get_zero_open_checkpoint_vbuckets code was preparing map from stat
names to vbuckets, and than matches stat name, but instead of grabbing
corresponding vbucket from map, it was parsing stat name that's we
just found equal to what we've built.

Change-Id: I826e72fd88a60c3ec365b54bd7cab9d5f69c2a10
Reviewed-on: http://review.couchbase.org/23595
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-5307: bucket warming up on some node(s) is not error

I.e. we're lowering log message severity which better reflects real
world.

Change-Id: I1a5d2e985a2f718ac8da5d2d6a3843b7a389f96d
Reviewed-on: http://review.couchbase.org/23596
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-5307: allow 'rebalance' with specified vbucket moves

So that we can exercise some specific moves for sake of benchmarking
and testing. As well as some cases of near-disaster recovery.

Change-Id: I87e0378daf0668c7c88530ba2b6b6845e0871861
Reviewed-on: http://review.couchbase.org/23597
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>