Fix parsing of root URIs

Also add a test to ensure this doesn't break in the future.
Define and use HTTP validation utility functions in test suite
Add random flood test
Abort coroutine on chunk-size to hex-string conversion failure
Try harder to find the correct version of Lua libraries

This change ensures only Lua 5.1.x or LuaJIT 2.0.x series will be
found.  The query syntax supported by FindPkgConfig.cmake isn't that
flexible enough to look only for libraries with versions within certain
values; the workaround is to look for the same package twice,
specifying the minimum version and the maximum version.

This hopefully puts an end to compilation woes regarding Lua library
versions.  This could be easily solved by having API-versioned .pc
files.