Clear last pool work on switching pools if the current pool supports local work generation or we are in failover only mode.
Merge pull request #395 from kanoi/bflsc

configure - set USE_USBUTILS when usbutils is required and use it in the code
Instead of keeping track of when the last work item was generated to keep stratum connections open, keep them open if any shares have been submitted awaiting a response.
Merge pull request #397 from kanoi/bflsc

usbutils.c copy full size to 'Last Command'
Make sure to retry only once with noresume support for stratum.
Implement client.show_message support for stratum.
Merge pull request #398 from kanoi/bflsc

API use control_lock when switching pools + Setup BFLSC support + API add 'Network Difficulty' to 'coin'
Cope with out of order messages coming in upon attempting to auth_stratum.
Make stratum connection interrupted message higher priority to be visible at normal logging levels.
Show mechanism of stratum interruption if select times out.
Close any existing stratum socket if we are attempting to restart stratum so the pool knows the connection has gone.