Updating to capnproto v0.8.0

- Includes some test stabilization
- Fixes manylinux2010 build issues (linker flag order due to old gcc)
- More rigorous python setup.py clean
- Requires capnproto v0.8.0 or greater
- Including system libcapnp include path for import (e.g. import
  stream_capnp)
- Bundle libcapnp .capnp files when not using system libcapnp
- Removing more distutils usage. Now using pkg-config to determine the
  system version of libcapnp (mainly for Linux, but should work on macOS
  with brew)
- Removed dead code

Resolves issues #215 #216 #217
Lots of fixes for Issue #218 (all sorts of retry methods needed for
GitHub Actions)

.github/workflows/manylinux2010.yml

@@ -24,6 +24,8 @@ jobs:
         /opt/python/${{ matrix.python-version }}/bin/python -m pip install -r requirements.txt
         /opt/python/${{ matrix.python-version }}/bin/python -m pip install auditwheel
     - name: Build pycapnp and install
+      env:
+        LDFLAGS: -Wl,--no-as-needed -lrt
       run: |
         /opt/python/${{ matrix.python-version }}/bin/python setup.py build
     - name: Packaging