* Throttle async session-activity writes to once per token per minute
* Add singleflight to session lookups, keystore validation and
column/row security loads to stop cold-cache stampedes
* Preload security rules in BeforeHandle (restheadspec, resolvespec) so
they no longer need a second connection while the read tx is open
* Add pkg/dbtrace: opt-in per-request DB call counting and pool logging
(db_trace.* config, RESOLVESPEC_DB_TRACE_* env), wired into testserver
* Add tests for load dedup, activity throttle and dbtrace
