Race conditions

[wch]

When running a (multithreaded) httpuv test app with valgrind + helgrind, it says there are possible races in the following:

• Simultaneous calls to set_fd
• Timestamp::diff_secs() and TimestampImplPosix::~TimestampImplPosix()

There are also some minor locking/unlocking issues when quitting R. Their effects are probably unimportant though.

Full output here:
https://gist.github.com/wch/9818a0321782c4e3cfcf321ee9a87cdb