Added a tentatively working implementation of MCAS.
doctest correction
Add MCAS load test; bug fixes.
Handle race condition where MCAS record can't be dereferenced.
mcas bug fixes:
   * failing to load mcas record for cas
   * nested mcas race
   * mcas record collection race
Change mcas / mcas_get to work with immutable Item object, avoiding bug-prone
interaction of implicit CAS ID's (e.g. from _mcas_help side effects).
simple unit tests for mcas
Make mcas work with App Engine client.
Rename mcas API Item to Entry.
Rework docs, adding mcas.
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