Lazy pool initialization

• Lazy pool initialization. pool.ts no longer eagerly constructs the
  pg-pool at module import. The pool is now created on the first call to
  execute() or SQLEntity.query.sync(). The new Pool(...) call is deferred.
• Fixes zombie boot on init failure. Prior to 0.22.0, the library eagerly
  opened a pg-pool handle that kept Node's event loop alive indefinitely.
  In case of issue the service could log a "cannot start" message then hang
  instead of exiting — because the pool's open handles blocked event-loop drain
  and no signal handler was registered on the boot path. Boot-failure paths are
  now free of antity-pgsql-attributable handles, so process.exitCode = 1
  strategies work again. Consumers using @dwtechs/servpico-express should
  continue to prefer .catch(failFast) for the explicit exit + stderr flush.
• Declares sideEffects: false. Now that module load is genuinely
  side-effect-free, bundlers can tree-shake unused exports. Consumers that only
  use the query-builder surface (SQLEntity.query.select, filter) without
  ever calling execute() never construct a pool at all.