User activity logs

Some customers want to keep detailed command logs executed by their users due to their security audit policies.Let's add an option to store all query-mode and batch-mode code snippets.

1st phase:

- [ ] Add an etcd configuration option
- [ ] Add the `activity_logs` database table
- [ ] Add an admin API to retrieve activity logs
2nd phase:

- [ ] Extend jail to log system call arguments and return values\* This will incur non-negligible performance overheads. We should implement this very efficiently, and buffer & compress the logs on the fly as hundreds of syscalls may be issued per second.

JIRA Issue: BA-334