[4.4] Allow speculative majority reads in multi-doc txns when --enableMajorityReadConcern=false

Affects 4.4 (observed on 4.4.17-rc0), doesn't seem to affect 4.2 (doesn't reproduce on 4.2.22). Doesn't affect 5.0+ as support for majority read concern is always on.

Behaviour:

• Multi-doc txn with txnNumber:0 and readConcern: { level: 'majority' }

  returns "Given transaction number 0 does not match any in-progress transactions".

  • I believe it should return "Majority read concern is not enabled" instead.
• Multi-doc txn with txnNumber:0 and readConcern: { level: 'majority' }

  and allowSpeculativeMajorityRead: true returns "Given transaction number 0 does not match any in-progress transactions".

  • I believe it should return "Majority read concern is not enabled" instead.
• Multi-doc txn with txnNumber: 1 and readConcern: { level: 'majority' }

  : aborts with "Majority read concern is not enabled"

  • This is expected.
• Multi-doc txn with txn txnNumber:1 and readConcern: { level: 'majority' }

  and allowSpeculativeMajorityRead: true crashes the server.

  • I believe it should return "Majority read concern is not enabled" instead.