fix a System.Buffers break via System.IO.Pipelines; 4.7.2 adds a netf…

…x TFM, which in turn adds System.Buffers breaks

- we now only take a dependency on 4.7.1, which means we don't *force* the issue on people
- Pipelines.Sockets.Unofficial has been updated to identify the problem clearly

avoid the array (multi-bulk) allocation in sentinel processors (Stack…

…Exchange#1491)

* avoid the array (multi-bulk) allocation in sentinel processors

* release notes

fix build

release notes

2.1.0 Release! (StackExchange#1394)

This bumps versioning for the 2.1.0 release, ready to GA!

add tracking for "now" and "next" messages when recording message tim…

…eouts

release notes for 2.0.593

release notes tweak

more release notes; this is *probably* r571

SDK: switch to 2.1.500

Until we can figure out how to install on Ubuntu...