MFC r254252:
Fix the formatting of the error message.

The G_MIRROR_DEBUG() macro already appends a newline. Also, most of the
log messages emitted by gmirror start with an uppercase letter.
MFC r256603:
Introduce new function devstat_end_transaction_bio_bt(), adding new argument
to specify present time.  Use this function to move binuptime() out of lock,
substantially reducing lock congestion when slow timecounter is used.
MFC r256606:
Move g_io_deliver() out of the lock, as required for direct dispatch.
Move g_destroy_bio() out too to reduce lock scope even more.
MFC r256610:
Add unmapped I/O support to GEOM RAID.
MFC r256607, r259247:
Fix passing uninitialized bio_resid argument to g_trace().
MFC r265054:
Reduce number of opens by REOM RAID during provider taste.

Instead opening/closing provider by each of metadata classes, do it only
once in core code.  Since for SCSI disks open/close means sending some
SCSI commands to the device, this change reduces taste time.