Add profile target to makefile
Add shutdown API command
Implement Engine::submitCommit()

This function is functionally equivalent to submit() followed by
commit() but performs better. It writes only a single entry to the
write-ahead log and is executed atomically, i.e. submit()s after
submitCommit() do not have to check for conflicts with the
submitCommit()ed transaction.

Use submitCommit() unless you have to do distributed transactions.