Create buffers for remote collaborators out of band

Previously, we would use `Project::serialize_buffer_for_peer` and
`Project::deserialize_buffer` respectively in the host and in the
guest to create a new buffer or just send its ID if the host thought
the buffer had already been sent.

These methods would be called as part of other methods, such as
`Project::open_buffer_by_id` or `Project::open_buffer_for_symbol`.
However, if any of the tasks driving the futures that eventually
called `Project::deserialize_buffer` were dropped after the host
responded with the buffer state but (crucially) before the guest
deserialized it and registered it, there could be a situation where
the host thought the guest had the buffer (thus sending them just the
buffer id) and the guest would wait indefinitely.

Given how crucial this interaction is, this commit switches to creating
remote buffers for peers out of band. The host will push buffers to guests,
who will always refer to buffers via IDs and wait for the host to send them,
as opposed to including the buffer's payload as part of some other operation.

crates/rpc/src/proto.rs

@@ -86,6 +86,7 @@ messages!(
     (RemoveContact, Foreground),
     (ChannelMessageSent, Foreground),
     (CopyProjectEntry, Foreground),
+    (CreateBufferForPeer, Foreground),
     (CreateProjectEntry, Foreground),
     (DeleteProjectEntry, Foreground),
     (Error, Foreground),
@@ -222,6 +223,7 @@ entity_messages!(
     BufferReloaded,
     BufferSaved,
     CopyProjectEntry,
+    CreateBufferForPeer,
     CreateProjectEntry,
     DeleteProjectEntry,
     Follow,