OpTracker: Sharding logic is implemented to improve performance

Enabling OpTracking is degrading performance significantly. The
main reason for this is the contention in the ops_in_flight_lock
during registration/unregistration. Sharding this lock and hence
the xlist (storing the ops) helped to boost the performance.
Sharding is been done based on the op seq number % num_shards.
atomic64_t is used as data type to protect the global sequence
number of OpTracker. Introduced a lock in the OpHistory to
protect its data structure as ealier it was protected by global
ops_in_flight_lock. This is only accessed during unregistration
and thus not contended.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
OpTracker: Removed _dump_op_descriptor_unlocked call for log level < 5

_dump_op_descriptor_unlocked call within the function _mark_event()
was invoked unnecessarily every time for all log levels. This was
degrading performance significantly! The stream is only used during
log level 5 and thus the _dump_op_descriptor_unlocked() is only called
now if user sets the log level to 5 and beyond.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
OpTracker: The TrackedOp::events list was not protected while dumping ops

OpTracker::Mutex::lock is there to protect the event list and it is
only used in the mark_event while pushing the events in the list.
It is not used while dumping the contents of the events list.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
OpTracker: Race condition removed while dumping ops through admin socket

OSD was crashing due to a race condition while IO was going on and
user wants to dump in flight ops. This was happening because Message
data and payloads are removed before the op itself is removed from
the in flight list. Calling op->_unregistered() after removing
the op from the in flight list fixes the issue.

Fixes: #9384

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Merge pull request #2440 from somnathr/wip-optracker-optimization

Wip optracker optimization

Reviewed-by: Samuel Just <sam.just@inktank.com>