separate `src make` and `src makefile` commands
simplify walkable FS usage
Don't check for BuildDir until necessary
fetch source unit build data files in parallel
build-data and push/pull commands using simplified build store
simplify build data store
`src build-data fetch/upload` and `src remote import-build-data` for pushing build data
re-add `src push` command
set build StartedAt and Host
Merge pull request #81 from sourcegraph/remote-builders

Remote builders