coap timing improvements

Description

Some updates to CoAP timing to address the overflow issue

• CoAP observables can optionally buffer incoming messages and only process the latest one
• CoAP client has non-zero sleep to reduce processor usage (made possible by updated coapsharp-barrett.dll version 0.2.0, see burt/software/coapsharp-barrett!2)

What to focus on

The functionality of BurtSharp programs should be unchanged by this merge request; none of the BurtSharp programs should be using the CoAP buffering. Compare wireshark capture to devel version to confirm timing is not adversely affected. Use the command line program htop to check processor usage (should be slightly reduced with this version).

The buffering should be tested with the burt-sharp-unity merge request burt/software/burt-sharp-unity!94

Resources

Thread signaling and locking: http://www.albahari.com/threading/part2.aspx

Issues

• Addresses burt/project#396