* lite2: add Start method
There are few reasons to do that:
1) separation of state and dynamics (some users will want to delay
starting the light client; does not matter we should not allow them
to create a light client object)
2) less important, but some users might not need autoUpdateRoutine and
removeNoLongerTrustedHeadersRoutine routines
* lite2: wait till routines are finished in Stop
because they are started in Start, it feels more natural to wait for
them to finish in Stop.
* lite2: add TrustedValidatorSet func