bladeRF-cli: rx/tx now use async interface, libbladeRF: USB dev speed API call

The rx and tx CLI commands now use the asynchronous stream interface to
yield more acceptable performance, and this code has been overhauled for
better readability. Command parameters have been added to allow for
configuration of the stream.

Although a bit overzealous, as "lock it if you use it" approach is taken
here. Optimizations may follow later..

Some rx/tx parameters now support K/M/G suffixes, where relevant.

To support sample rate sanity checks, an API function to check the
current USB speed has been added to libbladeRF. This speed is now
reported in the CLI 'info' command.