Synchronize gmirror(8) with HEAD:
- Speed up synchronization process by using configurable number of parallel
I/O requests.
- Allow for kernel dumps to the mirror.
- Reimplement gmirror softc synchronization by using per-device sx(9) lock.
- Stop synchronization from pre-sync hook.
- Fix possible race between synchronization and regular requests.
Approved by: re (mux)
c2dead3970