This places all the flag definitions together, making them easier to
find, and makes it possible (in the future) to use db for convenience
functions when we have a Go API, without having flag space contaminated
by cayley main flags.
Given that there may be other Turing complete query interfaces
(particularly a Go query API), the timeout config should not be
specifically tied to gremlin.
Having a time.Duration measuring seconds is likely to cause problems at
a later stage. This change retains configuration compatibility while
adding idiomatic duration use.
