Add message-db Store

[nordfjord]

Implementation of a MessageDB storage backend for Equinox.

Notable exclusions:

• MessageDB does not support reading stream backwards, for this reason there is no RollingSnapshots Access strategy provided

[CLAassistant]

All committers have signed the CLA.

[bartelink]

wont affect the IL but this can be a val too now ;)

[bartelink]

This looks done done to me ;)

[bartelink]

Thank you for the work, and for making reviewing it a pleasure!

[bartelink]

This is an oversight on my part... This needs to be streamVersion+1 due to

equinox/src/Equinox/Decider.fs

Lines 269 to 273 in 4b07909

	/// Exposes the underlying Store's internal Version for the underlying stream.
	/// An empty stream is Version 0; one with a single event is Version 1 etc.
	/// It's important to consider that this Version is more authoritative than counting the events seen, or adding 1 to
	/// the `Index` of the last event passed to your `fold` function - the codec may opt to ignore events
	abstract member Version : int64

This mess is due to the historic 'arguable' misdesign of having a stream with one event be 'Version' 0 in ESDB (that and the empty vs not created vs no events magic values).

(Cosmos and Dynamo use 0 for an empty stream - i.e. the version is events.Length)

If MessageDb is -1 based, then the chances are the only change required is adding 1 to the internal value (and adding the warning). If it's not, then we should take this opportunity to use 0-based indexes to make it all less mindbending.

https://github.com/jet/equinox/blob/master/src/Equinox.SqlStreamStore/SqlStreamStore.fs#L298

The other obvious thing here is that the tests should have caught this...
using Decider.QueryEx, the Version can be obtained by supplying a mapResult