cntl: parse fractional timeout as double rather than float

This might fix some spurious test failures where the expected result is
slightly truncated on some platforms

Change-Id: Ifc4bd1d8eeef770ec1b7d54905268d06563b7607
Reviewed-on: http://review.couchbase.org/41614
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>