BUG-5280: add the concept of a recorded failure

BUG-5280: add the concept of a recorded failure

This patch reworks LocalReadWriteProxyTransaction to be defensive
of its internal modification and introduces the concept of a delayed
recorded error (currently unused).

The defensiveness checks allow us to get rid of FailedDataTreeModification,
as we do not give out our modification at all in the codepaths which
would leak this implementation.

Change-Id: I5f91218ac308f7450a3b59252d44f953be54626c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>