Support graceful shutdown

[oleg-jukovec]

The graceful shutdown protocol is a mechanism that helps to prevent data loss in requests in case of a shutdown command.

https://www.tarantool.io/en/doc/latest/dev_guide/internals/box_protocol/#graceful-shutdown-protocol

[R-omk]

#119 this should be implemented first.

Details tarantool/doc#2633

[ice-ares]

what about supporting graceful shutdown from the client? Especially in the connection pool. I.E. client called .Close() or context cancelled, but we still have pending requests

[oleg-jukovec]

This is planned to be done by 2023. Perhaps it will be ready this month.