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)

examples/thread_client.py

@@ -30,7 +30,7 @@ class StatusSubscriber(thread_capnp.Example.StatusSubscriber.Server):
 
 
 def start_status_thread(host):
-    client = capnp.TwoPartyClient(host)
+    client = capnp.TwoPartyClient(host, nesting_limit=64)
     cap = client.bootstrap().cast_as(thread_capnp.Example)
 
     subscriber = StatusSubscriber()
@@ -39,7 +39,7 @@ def start_status_thread(host):
 
 
 def main(host):
-    client = capnp.TwoPartyClient(host)
+    client = capnp.TwoPartyClient(host, nesting_limit=64)
     cap = client.bootstrap().cast_as(thread_capnp.Example)
 
     status_thread = threading.Thread(target=start_status_thread, args=(host,))