Add KSTAT_TYPE_TXG type

Add KSTAT_TYPE_TXG type

Add a new kstat type for tracking useful statistics about a TXG.
The new KSTAT_TYPE_TXG type can be used to tracks the following
statistics per-txg.

  txg          - Unique txg number
  state        - State (O)pen/(Q)uiescing/(S)yncing/(C)ommitted
  birth;       - Creation time
  nread        - Bytes read
  nwritten;    - Bytes written
  reads        - IOPs read
  writes       - IOPs write
  open_time;   - Length in nanoseconds the txg was open
  quiesce_time - Length in nanoseconds the txg was quiescing
  sync_time;   - Length in nanoseconds the txg was syncing

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