Enforce stricter offset parsing

[pjenvey]

A followup to #368: Python syncstorage has stricter handling of offset values, raising an InvalidOffsetError when:

• a special "offset token" (timestamp_bound:offset format) is specified for sort=index
• a regular offset (just a plain numeric offset value) for other sort types

The change to db-tests' get_bsos_limit_offset in #368 somewhat came across case #1: the fact that passing a timestamp bound to sort=index is bogus.

Allowing case #2 made sense for the sake of deploying #368 (so a client w/ a plain offset retrieved from just before its deployment would continue working after the deployment). Now that it's deployed we could begin rejecting it also.

[pjenvey]

@fzzzy did most of this in #494 but we're postponing it for now until we decide how to handle Spanner's usage of the encoded offset (or lack thereof)

[pjenvey]

Dependent on #645