lib: timings - added quantiles

lib: timings - added quantiles
Just sub-sample the stream. On the assumption that the samples come from
one distribution, then any randomly selected subset will share the same
distribution. Therefore the quantiles should be at approximately the same
value.

However, that's a big assumption, as there will almost certainly be time
dependency, and periodicity (24hrs, 7 days).

Signed-off-by: Phil Carmody <phil@dovecot.fi>