Release 3.1.0

[WIP] use vcpkg for getopt; build windows shared lib enhancements (#350)

* fix MSVC compiler shared library issues - mostly around 'extern const'

* add vcpkg install getopt to appveyor-windows.yml

show appveyor where to get getopt from vcpkg (non-static lib to avoid LGPL violation)

* add missing else case in lsquic_shared_support.h for windows static lib path

* have cmake spit out it's version

have cmake copy dependent dlls to build dir for tests on windows (getopt.dll)

* copy getopt.dll dep for tests

added commented version that requires >= 3.21 but handles any dll deps

* try caching boringssl dir to reduce CI build time since it's always same commit specified in config file

define VCPKG_ROOT in env since I can't seem to find it by VCPKG_ROOT or VCPKG_INSTALLED_DIR in appveyor's cmake v3.16 + vcpkg

* make windows cache dependent on yml and cmd

* sync up with changes to ls-qpack

Release 2.27.1

- [API] New knob to set outgoing packet batch size.
- Aborted connection now become tickable immediately.
- Abort connection when HTTP/3 frame cannot be opened (can only happen
when malloc fails).

Release 2.26.2

- [BUGFIX] Do not drop incoming data when STOP_SENDING is received.
- [BUGFIX] Receipt of STOP_SENDING should not cause read-reset.
- [BUGFIX] Allow stream writes after receiving RESET.
- [BUGFIX] Typo in stream: ANDing enum with wrong flag.
- [BUGFIX] Reset elision: do not use zero as special stream ID value,
for zero is a valid stream ID in IETF QUIC.
- [API] Add optional on_conncloseframe_received() callback.
- Use zero error code in RESET stream sent in response to STOP_SENDING.

Release 2.19.0

- [FEATURE] DPLPMTUD support. IETF connections now search for the
maximum packet size, improving throughput.
- [DEBUG] Record event in stream history when on_close() is called
in dtor.

Rename test/unittests to tests/ and test/ to bin/