MB-20725: LWW monitoring

MB-20725: LWW monitoring

Add a number of new stats to the bucket stats and vbucket-details.

Add drift +/- threshold config parameters.

engine stats:

1. The maximum absolute cummulative-drift of the active vbuckets. See
    vbucket-details total_abs_drift
        ep_active_hlc_drift

2. The number of updates applied to ep_active_hlc_drift
        ep_active_hlc_drift_count

3. The maximum absolute drift of the replica vbuckets
        ep_replica_hlc_drift

4. The number of counts applied to ep_replica_hlc_drift
        ep_replica_hlc_drift_count

5. The total number of times a setMaxCas was from a peer who is
   ahead of the ahead threshold (hlc_ahead_threshold_us).
        ep_active_ahead_exceptions

6. The total number of times a setMaxCas was from a peer who is
   behind of the behind threshold (hlc_behind_threshold_us).
        ep_active_behind_exceptions

7. The total number of times a setMaxCas was from a peer who is
   ahead of the ahead threshold (hlc_ahead_threshold_us) for
   replica VBs.
        ep_replica_ahead_exceptions

8. The total number of times a setMaxCas was from a peer who is
   behind of the behind threshold (hlc_behind_threshold_us) for
   replica VBs.
        ep_replica_behind_exceptions

vbucket-details stats:

1. The current max_hlc (reported as max_cas for compatibility with
   other modules)
        vb_n:max_cas

2. The vbucket's absolute cummulative drift
        vb_n:total_abs_drift

3. How many updates have been applied to total_abs_drift
        vb_n:total_abs_drift_count

4. How many times the ahead threshold has been exceeded.
        vb_n:drift_ahead_threshold_exceeded

5. How many times the behind threshold has been exceeded.
        vb_n:drift_behind_threshold_exceeded

6. How many logical clock ticks this vbucket's HLC has returned
        vb_n:logical_clock_tick

Change-Id: I063782d4451b97f58a3c89208506bd8bd08b705e
Reviewed-on: http://review.couchbase.org/68158
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>