Problem: unnecessary extra thread for TDBE<->chain-abci exchanges

[tomtau]

instead of current two threads ("chain-abci" and "persistence"), it could just be one. instead of taking Storage, it'd take some subset (some new definition) that can do the lookups (like ReadOnly) + only modify certain parts (exposed by some methods) and immediately flush.
enclave protocol will need extra definitions for this unified TDBE<->chain-abci communications
and the other enums/structs (IntraEnclaveRequest for TVE, EnclaveRequest for TQE...) can be renamed to better reflect which enclave they are meant to interface with