rdbms: refactor caching in LoadBalancer::getReadOnlyReason()

rdbms: refactor caching in LoadBalancer::getReadOnlyReason()

Avoid nesting of the same getWithSetCallback() cache updates.
Also favor accuracy over initial cache use for the case where
there is already a master connection. Add missing "lockTSE"
flag to protect against stampedes updating stale values.

Change serverIsReadOnly() to use SELECT in mysql instead of
SHOW to avoid internal temporary tables.

Bug: T227838
Change-Id: I2b0d680c9c3bdc7aaa1d1e1d6beb2dd203a815f1