When querying all branch aliases in a snapshot, the underlying database query can
timeout as it is not properly indexable. That diff intends to mitigate that issue.
A first commit moves the branches filtering by target type client side to avoid
sending the costly request that might timeout.
Note that even it seems that all branches of a snapshot are iterated, most of
swh snapshots contains a single branch alias named `HEAD` that will be iterated
first. Branches iteration will stop once all aliases processed so in that case
the iteration will stop quickly.
A second commit puts processed snapshot sizes in cache to avoid sending the same
set of database queries each time a page is loaded when browsing the archive
in the context of a specific snapshot.
Closes T2734