The two hooks being added are:
void HookLogInit(const std::string& writer, const std::string& instantiating_filter, bool local, bool remote, const logging::WriterBackend::WriterInfo& info, int num_fields, const threading::Field* const* fields);
which is called when a writer is being instantiated and contains
information about the fields being logged, as well as
bool HookLogWrite(const std::string& writer, const std::string& filter, const logging::WriterBackend::WriterInfo& info, int num_fields, const threading::Field* const* fields, threading::Value** vals);
which is called for each log line being written by each writer. It
contains all the data being written. The data can be changed in the
function call and lines can be prevented from being written.
This commit also fixes a few small problems with plugin hooks itself,
and extends the tests that were already there, besides introducing tests
for the added functionality.
