[Legacy Sync] Fix offline upload delay

[stevensJourney]

Overview

Currently the Legacy sync implementation can cause delays for uploading CRUD operations made while offline.

This can happen if:

• CRUD operations are made while the client is offline
• The PowerSync client connect method is then called.
• The client connects to the service, but we don't currently immediately upload the pending operations.
• The client will download updates from the service, but wont apply them until we have uploaded all pending items (this has been delayed/ not triggered)
• The client is effectively blocked until the next keepalive event is received from the service (this kicks off a upload)

This PR triggers an upload after receiving the first PowerSync service command line. This same method is currently used by the Rust implementation.

The handles write made while offline test has been updated. We no longer send a keepalive immediately from the mocked service. This test would previously fail without the keepalive.

[simolus3]

Looks good to me 👍