don't let a web console page hang on a stopped service

[pobregat0]

The console asks other threads for a snapshot of their sessions and waits for the answer without any bound: GetECIESx25519SessionsList().get() for a destination and GetSSU2Sessions(sessions).get() for the transport. A task posted to a service that is already stopped never runs, so the waiting console thread would block forever and the page would never come back.

Both waits are now bounded by three seconds. If the answer does not arrive in time, the page says so in that place and renders the rest as usual instead of hanging.

This is what was asked for in the channel: call wait_for instead of get.

Checked locally: the router starts and the front page, local destinations, transports and tunnels pages all render in about a millisecond. Builds without warnings, unit tests pass, C++17 syntax check is fine.