HLTResultMTByteStreamCnv: Don't use FullEventAssembler and implement local cache instead
Stop using the hidden cache and the overcomplicated infrastructure of FullEventAssembler and instead:
- implement an explicit simple local cache
- create ROBFragments and append them to RawEventWrite (aka FullEventFragment) directly
A similar solution will be implemented for L1 results in !30902 (merged)