Resending pending subscriptions can result in inconsistent sync progress

[sync-by-unito]

When a change of query (COQ) needs to be sent to the server, the sync client first uploads the local changes up to the client version associated to the query. Sometimes, these upload messages are empty. If the the client disconnects and reconnects before the query was fully bootstrapped, the sync client re-sends the upload and COQ messages to the server. This is a problem especially if the upload message is empty. Due to history trimming, the sync client may not know anymore the last_integrated_server_version associated with the client version sent in the upload message. This results in the server issuing a Bad server version error (and a client reset) due to inconsistent sync progress (same client version is sent with two different server versions).

There are (at least) two solutions to this issue:

1. Skip sending empty upload messages before a COQ
2. Do not trim history past the latest client version acknowledged by the server

First solution is going to be implemented (this is also an optimization since the server does not really need these empty messages).