Disable duplicate JSON access logger in the default configuration

[vharseko]

Summary

New installations ship with two enabled access loggers: the text
File-Based Access Logger and the common-audit Json File-Based Access Logger (logs/ldap-access.audit.json), so every operation is logged twice.
Profiling after #660 shows the JSON handler's bounded write queue is the
single largest blocking point on the operation hot path under concurrent
load. This PR disables the JSON access logger by default for new
installations (it remains available via dsconfig) and documents it in the
admin guide. Existing installations are unaffected.

Problem

With the AuthenticatedUsers lock removed (#660), JFR jdk.ThreadPark
profiling of a saturated BIND workload shows worker threads spending most of
their blocked time waiting to enqueue log records:

blocking point	park events	total blocked (80 s window, 16 workers)
JsonFileWriter.put (JSON access logger)	3,535	52.0 s
AsynchronousTextWriter.writeRecord (text access logger)	1,928	32.3 s
com.sleepycat.je.latch.LatchImpl.acquireExclusive (JE, for scale)	1,352	37.2 s

Benchmark

Environment: packaged server built from this tree, je backend, 5,000 users
(uid=user.N, {SSHA} passwords) via setup --sampleData 5000, JDK 11,
8-core host (server and load generator co-located), 2 GB heap.

Load: authrate from the LDAP toolkit — 200 persistent connections, each
re-binding as a random distinct user; 30 s warm-up, 90 s measured,
steady-state = mean of the last 30 s:

authrate -h 127.0.0.1 -p 1389 -c 200 -f -B 30 -d 90 -i 5 \
  -D "uid=user.%d,ou=People,dc=example,dc=com" -w password \
  -g "rand(0,5000)"

Profiling: jcmd <pid> JFR.start settings=profile (JavaMonitorEnter /
ThreadPark / ExecutionSample) + periodic jstack sampling.

configuration	binds/s	resp mean	p99.9
default (text + JSON loggers on)	31,900	6.28 ms	40.1 ms
JSON logger off (this PR)	36,320 (+14%)	5.52 ms	25.7 ms
both access loggers off (tuning reference)	39,286 (+23%)	5.10 ms	22.4 ms

Errors were 0 in all runs.

Change

• opendj-server-legacy/resource/config/config.ldif: ds-cfg-enabled: false for cn=Json File-Based Access Logger, with a comment explaining
  why and how to enable it.
• Admin guide (chap-monitoring.adoc): the JSON handler is now listed among
  common audit handlers, and a new procedure "To Enable JSON LDAP Access
  Logs" covers enabling it with dsconfig (and optionally disabling the
  text logger to avoid double logging).
• Admin guide (chap-tuning.adoc): note in Logging Settings that each
  enabled access logger multiplies per-operation logging cost.

Testing

• Fresh install from the rebuilt package: list-log-publishers shows the
  JSON logger disabled and the text logger enabled; logs/access is
  written, logs/ldap-access.audit.json is no longer created.
• dsconfig set-log-publisher-prop --publisher-name "Json File-Based Access Logger" --set enabled:true re-enables it as documented.