commandserver: add IPC channel to teach repository path on command finished The idea is to load recently-used repositories first in the master process, and fork(). The forked worker can reuse a warm repository if it's preloaded. There are a couple of ways of in-memory repository caching. They have pros and cons: a. "preload by master" pros: can use a single cache dict, maximizing cache hit rate cons: need to reload a repo in master process (because worker process dies per command) b. "prefork" pros: can cache a repo without reloading (as worker processes persist) cons: lower cache hit rate since each worker has to maintain its own cache c. "shared memory" (or separate key-value store server) pros: no need to reload a repo in master process, ideally cons: need to serialize objects to sharable form Since my primary goal is to get rid of the cost of loading obsstore without massive rewrites, (c) doesn't work. (b) isn't ideal since it would require much more SDRAMs than (a). So I take (a). The idea credits to Jun Wu.