NVME trim clocking

Add the ability to set two goals for trims in the I/O scheduler. The
first goal is the number of BIO_DELETEs to accumulate
(kern.cam.XX.U.trim_goal). When non-zero, this many trims will be
accumulated before we start to transfer them to lower layers. This is
useful for devices that like to get lots of trims all at once in one
transaction (not all devices are like this, and some vary by workload).

The second is a number of ticks to defer trims. If you've set a trim
goal, then kern.cam.XX.U.trim_ticks controls how long the system will
defer those trims before timing out and sending them anyway. It has no
effect when trim_goal is 0.

In any event, a BIO_FLUSH will cause all the TRIMs to be released to
the periph drivers. This may be a minor overloading of what BIO_FLUSH
is supposed to mean, but it's useful to preserve other ordering
semantics that users of BIO_FLUSH reply on.

Sponsored by: Netflix, Inc