Support cartridge hot-reload

[rosik]

The first thing to be done is to implement the stop function. Despite the migrator role is permanent, Cartridge always calls stop + init callbacks and cleans HTTP routes upon hot-reload.

The second is the overall testing.

[no1seman]

Hot-reload of migrations also depends on https://github.com/tarantool/cartridge/issues/1328 because we need a guarantee that during mirgation hot-reload fully blocked.

[filonenko-mikhail]

Hot-reload of migrations also depends on tarantool/cartridge#1328 because we need a guarantee that during mirgation hot-reload fully blocked.

It is unblocked now

[no1seman]

Seems that it's not actual since we a going to add support of online-ddl to tarantool/ddl, but let this issue stay open for now....