DynamoStore: Cache decoding of Data/Meta

[bartelink]

Adds logic to correctly handle cases where the reader traverses the TimelineEvent.Data more than once.

In the typical case, Equinox-controlled loads will only walk the events exactly once [until isOrigin stops the search]
However in more exotic cases such as in projection loops, there can be cases (and this issue surfaced as an exception in such logic) where layering is such that filtering layers perform some pre-traversals.

It has been argued by @yreynhout that compression doesn't belong in this layer; certainly its absence would have avoided this specific issue, and I can't say I like the impl having ugly excuses and guards as this PR introduces.

Removing it would at a minimum entail adding some form of content-encoding/content-type to the ITimelineData in order for an outer layer to be able to drive it. Am considering doing it, likely with a breaking format change prior to entering the -rc phase.