leadership settings documents written out without env-uuid field

[Menno Finlay-Smits]

A bare empty map is written to the settings collection by addLeadershipSettingsOp which is unsupported by the multi-env txn layer. This means that the document is written out without the env-uuid field.

The lack of an env-uuid field causes the addEnvUUIDToEntityCollection upgrade helper to do the wrong thing when working on the settings collection, adding a env UUID prefix to _id field when there's already one there.

Fixes:

1. addLeadershipSettingsOp should be updated to using bson.D/M so that the multi-env txn layer can automatically add the env-uuid field.

2. The multi-env txn layer should error out when it sees a document expressed with an unsupported type.

3. the addEnvUUIDToEntityCollection upgrade helper should not attempt to add an env UUID prefix to the _id if it's already there.

[Menno Finlay-Smits (menno.smits)]

Fix #1 is addressed by PRs 2661 and 2662.

[Menno Finlay-Smits (menno.smits)]

Fix #2 is addressed by PRs 2666 & 2667.

[Menno Finlay-Smits (menno.smits)]

Fix #3 is addressed by PRs 2670 & 2671