Don't send or respond to HTTP or MQTT requests while importing a database

[scottgonzalez]

trafficstars

When importing a database, the server will respond to requests before all platforms have been loaded. This means that in a competition with platforms A, B, and C, any requests for platforms B or C may get a response for platform A as soon as owlcms has processed enough to load platform A. The server should not respond to requests until all platforms have been loaded.