• Properly implement the symbol server protocol for requesting ELF and Mach-O files.
• Unfortunately this requires version bumping ClrMD to 2.1 because this is a breaking change.
• This means we now support cross-OS debugging for the scenarios that CLR Diagnostics supports. E.G. it's possible to debug Linux coredumps on a Windows machine for recent builds of .Net Core which support it.
• Theoretical support for OS X -> Windows debugging exists since we plan to build that eventually, but currently the CLR Diagnostics team has not built the dac PE image that can debug OS X dumps.
• Fixed some Linux and OS X build issues.
• Do a better job parsing symbol server paths.
• Better understand the kinds of symbol servers out there and make less requests that will never be filled.

Fixes #977.
Fixes #927.
Closes #771.