Now "Flamenco-Worker/{version}" is used, instead of
"Flamenco-Worker {version}", to comply with RFC 7231 section 5.5.3
https://tools.ietf.org/html/rfc7231#section-5.5.3

This header is included with each request, so that the Manager can handle
software version bookkeeping however it sees fit.

Round away the milliseconds, as those aren't all that interesting.
Uniqueness is ensured by calling _unique_path().

The colons in the timestamp are gone, and numerical suffixes (to ensure
new directories are unique) are prefixed with a ~.

If the keyboard interrupt is received while the "may I run" task is just
connecting to the Manager, it would cause an exception that would halt the
graceful shutdown.

This is a clone of the Flamenco repository from back in the days when
Server, Manager and Worker shared the same Git repository. This is the
commit where that ended, and they went their separate ways.

Go 1.8 introduced a way to cleanly shut down the HTTP server. This allows
us to handle any currently running requests while rejecting new ones, and
then fully shut down.

There were no formatting directives.

The 'done' channel and 'done_wg' wait group are now kept in a separate
struct. This allows for easier tracking & debugging of what's going
on, and allows for possible locking & other tricks to prevent race
conditions.