Add visibility in to txg sync behavior

Add visibility in to txg sync behavior

This change is an attempt to add visibility in to how txgs are being
formed on a system, in real time. To do this, a list was added to the
in memory SPA data structure for a pool, with each element on the list
corresponding to txg. These entries are then exported through the kstat
interface, which can then be interpreted in userspace.

For each txg, the following information is exported:

 * Unique txg number (uint64_t)
 * The time the txd was born (hrtime_t)
   (*not* wall clock time; relative to the other entries on the list)
 * The current txg state ((O)pen/(Q)uiescing/(S)yncing/(C)ommitted)
 * The number of reserved bytes for the txg (uint64_t)
 * The number of bytes read during the txg (uint64_t)
 * The number of bytes written during the txg (uint64_t)
 * The number of read operations during the txg (uint64_t)
 * The number of write operations during the txg (uint64_t)
 * The time the txg was closed (hrtime_t)
 * The time the txg was quiesced (hrtime_t)
 * The time the txg was synced (hrtime_t)

Note that while the raw kstat now stores relative hrtimes for the
open, quiesce, and sync times.  Those relative times are used to
calculate how long each state took and these deltas and printed by
output handlers.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>