Mempool could carry a single era block state always

The Mempool is virtually just a block on top of the tip + 1 slot, therefore it lives in a single era. The internal state could carry a single era ledger state + tables, which would make revalidation for the hard fork block faster, going from this:

```
Syncing with the ledger consists of:
1. Consult txins (blk -> hf)
2. Read backing store (hf)
3. Forward them through db-changelog (hf)
4. Attach result to tip (hf)
5. Tick (hf)
6. Repeatedly *REAPPLY* (hf)
1. Unwrap HF state (hf -> blk) -- injection/translation every time
2. Call ledger reapplyTx (blk)
3. Compute diff (blk)
4. Attach diff to Tracking (blk -> hf)
5. Inject new values to Tracking (blk -> hf)
7. Store diffs (hf)
```

To this:
```
Syncing with the ledger consists of: (NEW)
1. Consult txins (blk -> hf)
2. Read backing store (hf)
3. Forward them through db-changelog (hf)
4. Attach result to tip (hf)
5. Tick (hf)
6. Unwrap HF state (hf -> blk) -- injection/translation once
7. Repeatedly *REAPPLY* (blk)
1. Call ledger applyTx (blk)
2. Compute diff (blk)
3. Attach diff to Tracking (blk)
4. Inject new values to Tracking (blk)
8. Store diffs (blk)
```

This might not be necessary depending on #347