Turn `inMem` heuristic to bool cfg

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`

Start by tracing inMemExec, isMining, and txc.Doms to the type Sync definition and the three Sync instances. Read how StageState.CurrentSyncCycle.Mode is populated, then confirm the three modes cover Mining, ForkValidation, and ApplyBlocks, with InMem mapped to ForkValidation. Done means the heuristic is removed and all Sync instances use the explicit mode.

Written by the indexing model from the issue text.