We historically removed reading from the transaction log to get consistent
results from _GET calls. There was also the motivation that the read-modify-update
principle we apply should not be hidden from the user. We still agree on the fact
that we should not hide these aspects but the impact on updates is quite significant
especially if the same documents is updated before it's written to disk and made serachable.

This change adds back the ability to read from the transaction log but only for update calls.
Calls to the _GET API will always do a refresh if necessary to return consistent results ie.
if stored fields or DocValues Fields are requested.