test: deploy a cluster of 60 instances

This patch introduces a new type of tests - manual. They aren't run by default. You can use it for development experiments:

pytest test/manual -s -k instant
pytest test/manual -s -k chunked

Also there're several changes that make it work smoothly:

• fix: shrink default tarantool memtx_memory
• fix: adjust timeouts in posjoin
• fix: introduce timeout when conf_change fails
• fix: no join requests retrying
• chore: make UnknownRecepient error more verbose