Turn `inMem` heuristic to bool cfg

[AskAlexSharov]

need get rid of inMemExec := txc.Doms != nil

Let's turn inMemExec and isMining into enum with 3 options: Mining/ForkValidation/ApplyBlocks (feel free to propose better naming)

InMem - it's ForkValidation

Value let's store inside type Sync struct (we have 3 sync instances) and set to StageState.CurrentSyncCycle.Mode

[mh0lt]

I think we should rename Mining to BlockProduction

[AskAlexSharov]

oke BlocksProduction ForkValidation ApplyBlocks (or Sync or ?)