Use LongAdder for LDAPStatistics counters

[vharseko]

Summary

Part of the bind/compare hot-path series (#660, #667–#670, #672, #674): the
last shared-hot-cache-line point identified by both profiling passes.
LDAPStatistics is a single instance shared by every connection of a
connection handler (keep-stats is on by default), and its counters are
updated 7–9 times per operation of any type:

• updateMessageRead: messagesRead, operationsInitiated, per-type
  request counter;
• updateMessageWritten: messagesWritten, per-type response counter,
  operationsCompleted;
• updateOperationMonitoringData: per-type operation count + resident time;
• updateBytesRead/updateBytesWritten.

With AtomicLong every worker thread CAS-es the same few cache lines on
every LDAP message — the same pattern removed from AuthenticatedUsers
(#660), the password schemes (#667), getDefaultPasswordPolicy (#669),
CompressedSchema (#670) and AciList (#674).

Change

All 51 counters become final LongAdder: updates use increment()/
add() (striped cells, no shared CAS under contention), reads use
sum() and only happen when cn=monitor data is requested;
clearStatistics() uses reset(), which has the same
non-atomic-under-concurrent-writes semantics the previous set(0) had.
Public getter signatures are unchanged (long). getMonitorData()
passes explicit sum() values instead of live counter objects.

Performance note

Honest caveat, as with the rest of the series: on the 8-core benchmark
host the effect is not locally measurable — an earlier A/B with
keep-stats:false (removing the counters entirely) was within noise at
~40k ops/s. The value is structural: LongAdder removes ~7–9 shared-line
CAS per operation for every operation type, a ceiling that grows with
core count, at the cost of slower reads on the rarely-used monitoring
path — exactly the trade-off LongAdder is designed for.

Testing

mvn -P precommit -pl opendj-server-legacy verify for
CompareOperationTestCase (37), SearchOperationTestCase (75),
AbandonOperationTestCase (30), TestLDAPConnectionHandler (3) — these
suites assert exact counter values through the public getters:
145 tests, 0 failures.

Files

• opendj-server-legacy/src/main/java/org/opends/server/protocols/ldap/LDAPStatistics.java