Add timer functionality to the async IO framework.
Tweak timer implementation:

- Define separate times for durations vs. absolute times.
- Make more consistent with existing precedents in KJ style.
Merge branch 'master' of github.com:kentonv/capnproto
Fix build on OSX and also GCC 4.7.
Fix Makefile.am.
When an RPC method name matches a C++ keyword, add an underscore to avoid compilation errors.