to join a workspace, attach to it. to leave a workspace, close the
channel. on such channel you get workspace events (new buffers, user
leave, ...). must fetch current buffers and users upon join. if
workspace doesn't exist, server should create it on attach
also dashmap everywhere to get/put simple
reuse as much as possible, keep rpc messages close with their rpc,
helper struct for uuid with into() and from(). also replaced the simple
things, such as imports and struct fields
