roachtest: enable MVCC stats assertions more aggressively

[erikgrinaker]

We should assert MVCC stats in roachtests, by enabling the following environment variables:

• COCKROACH_CONSISTENCY_AGGRESSIVE=true
• COCKROACH_ENFORCE_CONSISTENT_STATS=true

This will perform synchronous consistency checks after certain operations, and crash the node if any MVCC stats are incorrect when compared to the real data.

This may surface latent stats issues, so we should be prepared to clean these up.

Jira issue: CRDB-18854

[blathers-crl]

cc @cockroachdb/replication

[tbg]

Whipped something up in #87106 but a manual run revealed that running the full checks tends to drive nodes into OOMs, so we'll need a few DB-side changes to make that work.

COCKROACH_ENFORCE_CONSISTENT_STATS should be in the cards, though.