Reconsider minimum boost version

I think it's time to reconsider and clarify the minimum version of boost that pion depends on.

boost 1.35 is eons old.
boost asio has changed from 'io service' to 'io context' and 'executors' in later versions.
@rimmartin has basically updated to boost 1.70 in commit e3571540dd04 (which even requires std::chrono for LogService).
I'd like to get rid of some warnings about implicit 'fallthroughs' using the BOOST_FALLTHROUGH macro (see #16), which is available since boost 1.54.
Explicit boost::bind placeholders (see #13) would need at least boost 1.39.0.
'Generic TLS' ssl context as of commit 9130e98efc88, which is available since boost 1.64 (boost asio 1.10.9)

rimmartin / pion-ng