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)

.gitignore

@@ -8,6 +8,8 @@
 *.egg-info
 dist
 build
+build32
+build64
 eggs
 parts
 bin
@@ -32,7 +34,7 @@ nosetests.xml
 .project
 .pydevproject
 
-# IntelliJ 
+# IntelliJ
 .idea/
 
 # Cpp files
@@ -47,3 +49,6 @@ capnp/lib/capnp.h
 bundled/
 example
 *.iml
+
+# capnp files
+*.capnp