Connections to doc are possible while `onLoadDocument` is still in progress

[jordangarcia]

Description
The issue arises when multiple client connections come in for the same document, the first connection will kick off and await the onLoadDocument hook, however subsequent connections (while its still loading) incorrectly receive a connection to an empty document. When paired with @hocuspocus/provider it changes the meaning on onConnect and onSync. For the first client both of those hooks imply that the document has been loaded. For the second connection it is ambiguous if the document has been loaded.

Steps to reproduce the bug

• client1 connects to doc
• client1 connection invokes createDocument
• this.documents.set(documentName, document) immediately happens
• client1 starts onLoadDocument async hook
• client2 connects, sees that this.documents.has(documentName) is true and receives an empty doc, and successfully connects to an empty doc
• client1 onLoadDocument hook resolves
• client1 successfully connects

Expected behavior

• client1 connects to doc
• client1 connection invokes createDocument
• client1 starts onLoadDocument async hook
• client2 connects, sees that onLoadDocument is still in progress
• client1 onLoadDocument hook resolves
• client1 successfully connects
• client2 successfully connects