Reuse All-Users repository handle for account caching

The accounts cache sits directly on the hot path of virtually all Gerrit
operations, including Git pushes/fetches and REST API requests.

Previously, account lookups and cache loads opened All-Users through the
repository manager. JGit can reuse cached repositories, but resolving
their FileKey still adds overhead when the project-to-FileKey cache is
empty or disabled with core.useFileKeyByProjectCache=false.

The preceding change introduced a provider that retains a shared
All-Users handle. Complete its adoption in the accounts cache by using
that handle without closing it in either lookups or cache loads. The
provider manages its lifetime and releases it at server shutdown.

Release-Notes: Improve account-cache performance by reusing All-Users.
Change-Id: I3dae6280fca75d9befdc62a3fc5fa994b20ff097