libstore: turn periodic gc attempt into a promise

notably we will check whether we want to do GC at all only once during
startup, and we'll only attempt GC every ten seconds rather than every
time a goal has finished a partial work call. this shouldn't cause any
problems in practice since relying on auto-gc is not deterministic and
stores in which builds can fill all remaining free space in merely ten
seconds are severely troubled even when gargage collection runs a lot.

Change-Id: I1175a56bf7f4e531f8be90157ad88750ff2ddec4

0 files changed, 0 insertions, 0 deletions