[Parallel Executor] [VM] Allow transaction execution to be suspended / resumed on reads.

[gelash]

Currently parallel execution abandons a speculative VM execution of a transaction immediately when it encounters a dependency (i.e. an entry marked as Estimate in the shared MVMemory data-structure). This is an optimization that avoids aborts, as the transaction is then re-executed after the dependency is resolved. However, the re-execution starts from scratch, when in theory, it could have continued exactly from the prior state when the dependency was observed.

Way to solve this could involve some continuations / async programming and managing suspended execution tasks alongside dependencies in executor/scheduler. Exact performance gains need to be investigated, but it's promising.