MB-100 Allow floating point purge interval.

Only for testing purposes. And the only way to actually set it will be
through /diag/eval trickery.

Change-Id: I7036d1b47fb858c5ff4e6bc175adb449b445a782
Reviewed-on: http://review.couchbase.org/27932
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>