HBASE-30348 Throttles should work for system tables (excluding quotas, meta, namespace)

RegionServerRpcQuotaManager#getQuota returned NoopOperationQuota for any table
where TableName#isSystemTable() is true, and QuotaCache#getUserLimiter returned
NoopQuotaLimiter under the same condition. That covers every table in the hbase
namespace and every table in the backup namespace, so no quota of any kind could
throttle them: the quota was accepted, stored, and displayed by list_quotas, but
the request path never consulted a limiter and nothing warned the operator.

Only hbase:meta, hbase:quota and hbase:namespace are exempt now. All three are
read while a master is still initializing, before quotas can be served, so
throttling them could prevent a throttle from ever being lifted: quotas load
from hbase:quota, clients resolve region locations through hbase:meta, and
TableNamespaceManager reads hbase:namespace during initClusterSchemaService,
which runs before initQuotaManager creates hbase:quota. An earlier attempt at
this change omitted hbase:namespace and deadlocked master startup on branch-2.x.

QuotaCache#ensureInitialized loaded the cache on the calling RPC handler, and
that load both reads hbase:quota and asks the master for cluster metrics. A read
of a table which is not exempt could therefore wait on a master that cannot
answer until the read itself completes. The load now runs on a separate thread
and the request waits for it with a timeout, configurable via
hbase.quota.cache.initial.load.timeout.ms and defaulting to ten seconds. A
request that waits out the timeout proceeds with default quota state, which does
not throttle, and the load is picked up once it finishes.

This keeps the load synchronous in effect, so a quota which was just set is
still visible to the next lookup and cluster scope factors are still computed
before the first cluster scope check. It also means requests no longer queue on
the monitor that the refresh holds while it talks to the master.

Adds TestQuotaUtil covering the exemption, including that hbase:namespace is
exempt while the backup tables and the other hbase namespace tables are not.