[grpc] support loading ACLs from ephs

[grpc] support loading ACLs from ephs

Supporting changes:

- Rearranged grpc into internal/{client,server,common} packages to deal with circular dep between ephs and grpc
- Rename `mtls.parseName` -> `mtls.ParseRemoteIdentity` and make the returned struct and its properties public
- In ephs server ACLs, try to parse principal; if successful, set `name`, `class` and `domain` substitutions in the key
- Fixed severely incorrect invocation of acl.Check in ephs server (not a security issue, legitimate requests were blocked but no illegitimate requests were allowed)
- Add `utils/context` package - wrapper for `context` with shared `Interruptible` context that cancels on SIGTERM and SIGINT