Asynchronous snapshot upload

[Horusiath]

Build on top of #547

This PR introduces async DB file snapshotting. Basically when checkpoint happens, we trigger asynchronous DB file backup. If the next checkpoint happens, it first waits until previous snapshot upload completes before continuing.

[psarna]

I'm on vacation next two weeks, so just to keep priorities straight - either this PR, or #547, need to be applied, so that we're 100% sure we don't have autocheckpoint enabled on connections, and only control the checkpoints ourselves. More importantly, in the fiber that performs the checkpoint, we need to make sure that it's performed from a connection that has our custom WAL methods registered, so that we call the on_checkpoint behavior for bottomless, implemented here:

sqld/sqld/src/replication/primary/logger.rs

Line 171 in bb27f19

fnon_checkpoint(

[Horusiath]

@haaawk we still need unlock from @psarna to merge this.

[psarna]

Looks like #547 is applied and turns off autocheckpointing where applicable