[FS-925] Lazily create an MLS Self-conversation

[mdimjasevic]

This is a follow-up to #2730.

It changes the introduced endpoint's method from PUT to GET. Furthermore, it creates the self-conversation on the first call to the GET endpoint, but such that the epoch column is not written to so as to avoid undesired race conditions of another client also creating the conversation and bumping the epoch number (the rest of the columns in the conversation record are identical, so there is no harm in overwriting such columns). The epoch number is actually written to the database only upon the first commit.

Checklist

• See if there are additional checks that should be done when inserting the creator client's key package ref
• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[pcapriotti]

Looks good modulo comments below.