Add milestone for activity before TS_EVENT_HTTP_SSN_START

#include <ts/ts.h>
#include <cstddef>

int globalHandler(TSCont continuation, TSEvent event, void * data) {
        TSHttpSsn session = static_cast< TSHttpSsn >(data);
        TSHttpSsnReenable(session, TS_EVENT_HTTP_CONTINUE);
        return 0;
}

void TSPluginInit(int argc, const char * * argv) {
        TSPluginRegistrationInfo info;
        info.plugin_name = const_cast< char * >(PLUGIN_TAG);
//      If we create a Mutex here there is huge, unaccounted for TTFB latency
      TSCont continuation = TSContCreate(globalHandler, TSMutexCreate());
//      the following will "fix" this problem assuming you don't actually need the mutex
//        TSCont continuation = TSContCreate(globalHandler, NULL);
        TSHttpHookAdd(TS_HTTP_SSN_START_HOOK, continuation);
}