Most of the previous usages of get_repository_for_tenant were followed
by immediately getting a timeline in that repository, without keeping it
around for longer.
The new `get_timeline_for_tenant` function implements that same
behavior, but in one line.
In order to exclude problems with synchronizing disk and memory logical
size is not stored in metadata on disk. It is calculated on timeline
"start" by scanning the contents of layered repo and then size is maintained
via an atomic variable.
This patch also adds new endpoint to pageserver http api: branch detail.
It allows retrieval of a particular branch info by its name. Size info
is also added to the response of the endpoint and used in tests.
Once upon a time, 'page_cache.rs' contained an actual page cache, but
it hasn't for a very long time. Rename to reflect what it actually does
these days.
