Thread-safety for transactional puts

[cjfuller]

Transaction objects in the Google datastore client are not threadsafe. While it doesn't seem like a common usecase to share a transaction object across threads or coroutines, we should add synchronization around puts in a transaction to be sure that this isn't a problem.

KA-internal issue: INFRA-2835

[samuelrey]

@corajr had recently experienced some friction using multiGet/multiPut from within a transaction [1]. The process of sending the RPC has the Google datastore spawn a new thread which is explicitly forbidden by the library.

[1] https://github.com/Khan/webapp/blob/f70dc92ef6051b0c53aee4acc0ff12f521629f80/dataflow/progress-reports/daily-activity-time/src/main/kotlin/DailyActivityTimePipeline.kt#L125-L141

[corajr]

I think the SSL handshake but I'm not entirely sure.

Re: 62aca94, we were having some threading issues in Dataflow DoFn (I believe due to an SSL handshake spawning a separate thread when the number of query results is high) and so the temporary workaround for it was to use the underlying Java library's runInTransaction method... I hope this isn't too objectionable, and I will revert it if so.