support memcache flags during snapshotting and replication

[romange]

memcache has additional flags property for each its entries. We maintain it in a separate datastructure mc_flags.

1. SAVE/LOAD procedures do not support them because it's not part of the standard Redis datastrcutures
2. Also the replication ignores them.

We have a user that runs Dragonfly in memcached mode and H/A and asks for those features to improve the availability of their deployment.

[royjacobson]

Is the journaling subsystem supporting memcache commands? We might need to fix this as well.

[Abhra303]

What is the latest update on the issue?

[sbkoto]

not fixed on 1.19.0

set item 4 3600 4
test
STORED
get item
VALUE item 4 4
test
END

after reload dragonfly and load from snapshot:

get item
VALUE item 0 4
test
END