feature: transactions with single-threaded execution context

[gvolpe]

fixes #681

This seems to be working reliably. It still requires more testing and thoughts on user experience, but reliability comes first. I will be away for a few weeks, so this is all I got until I'm back.

The new implementation is named RedisTx and there is an example named RedisTxDemo showing how it works. This is the expected output (or similar) when running it:

[info] running dev.profunktor.redis4cats.RedisTxDemo
test1: foo
test2: bar
multi: pool-412-thread-1
set: pool-412-thread-1
set: pool-412-thread-1
set: pool-412-thread-1
set: pool-412-thread-1
exec: pool-412-thread-1
test1: nix
test2: linux
keep doing stuff...
KV: Map(test1-v1 -> sad, test1-v2 -> nix)
19:13:34.580 [io-compute-2] INFO  d.profunktor.redis4cats.LoggerIOApp - Releasing Commands connection: redis://localhost
19:13:34.583 [io-compute-2] INFO  d.profunktor.redis4cats.LoggerIOApp - Releasing Redis connection: RedisURI(redis://localhost)

I added a method that shows the current thread where each operation runs (needs to be all the same thread), which will be removed before this PR gets merged.

The final KV: Map(...) is the result of the GET operations. This is the part that requires more UX thoughts.