chore: disk backpressure class utility

[kostasrim]

This PR adds a simple producer/consumer utility class that for a backing file F can:

• write/read blobs sequentially from disk

This class will be used by dragonfly connection to offload commands when dispatch queue surpasses a given value.

No functional change in dragonfly_connection -- I just added the initialization paths and the flags needed.

Resolves #6029

[kostasrim]

@dranikpg we changed direction and we will only push/pop raw data. The rationale behind this is that we want to unblock the ConnFb. See: https://github.com/dragonflydb/dragonfly/issues/5952

[romange]

I think there is still work that needs to be done. In any case, I suggest checking how it will fit once you integreate OnRecv and be able to offload/load inside DflyConn.