The docs say this can happen for a directory if there's an I/O error.

Briar crashed when run in a device without bluetooth or without camera
such as an emulator.

Closes #514

Fixes #572, #578.

Will currently fail at runtime; requires a public key and a server onion.

Modified the project structure, removed module extension and went instead for a non-complete core dependency graph

Semi-encapsulated the core/api dependency graphs and created a proper structure to load eager singletons

When more than one invitation socket is opened, Alice should pick which one to use and Bob should use whichever one Alice picks. This fixes a race condition where each party picked a different socket and closed the other.

If two contacts publish their descriptors simultaneously, they may both stop polling without retrieving each other's descriptors. Continue polling for 2 intervals after publishing the descriptor.