HTTP connections not immediately closed on `db.disconnect()`

[rkistner]

When calling db.disconnect(), the sync Isolate is closed, but the HTTP connection still remains open for a while.

This does not cause any consistency issues on the client, but this counts towards concurrent connections and operations synced on the server.

This is especially noticeable when connecting and disconnecting in a loop, which can cause concurrent connections on the server to keep on increasing for a couple of minutes.

[rkistner]

Fix released in v1.5.2