Support creating realms in "pure" in-memory mode

[astigsen]

Currently Core supports an in-memory mode, but it still temporarily uses a backing memory mapped file (it just never explicitly syncs it to disk). It does this so that multiple processes can concurrently work on the same in-memory dataset.

This is generally a good thing, but for running under wasm, memory mapped files become very problematic, and there is no need for multi-process support there anyways. So it would be very useful to support a "pure" in-memory mode that uses regular allocations rather than memory mappings.

[sync-by-unito]

➤ bmunkholm commented:

Waiting for feedback from Nabil before progressing further.

[sync-by-unito]

➤ nhachicha commented:

[~brian.munkholm@mongodb.com]  I exchanged with [~jorgen.edelbo@mongodb.com]  already on slack, I think the PR is ready to be merged. The C-API tests passing on WASM are based on this branch. There are a couple of issues (https://jira.mongodb.org/browse/RJS-2049  https://jira.mongodb.org/browse/RJS-2046 and https://jira.mongodb.org/browse/RJS-2048) still, but in my opinion, are unrelated to this in-memory branch... 

Also, can you share your thoughts on #6022 (comment) ?

[sync-by-unito]

➤ bmunkholm commented:

The changes are pretty many, although most are simple, but before merging I think this needs detailed reviews and tests to avoid regressions for file based use.