Removed usage of pthreads and replaced with std C++ features

- Replaced pthread_mutex_t in modsecurity::operators::Pm with std::mutex
- Replaced pthread's thread usage in reading_logs_via_rule_message
  example with std::thread.
  - Simplified and modernized C++ code.
- Removed unnecessary includes of pthread.h

src/collection/backend/in_memory-per_process.cc

@@ -25,8 +25,6 @@
 #include <memory>
 #endif
 
-#include <pthread.h>
-
 #include "modsecurity/variable_value.h"
 #include "src/utils/regex.h"
 #include "src/utils/string.h"

src/collection/backend/lmdb.cc

@@ -27,8 +27,6 @@
 #include <string>
 #include <memory>
 
-#include <pthread.h>
-
 #include "modsecurity/variable_value.h"
 #include "src/utils/regex.h"
 #include "src/variables/variable.h"

src/collection/backend/lmdb.h

@@ -27,12 +27,10 @@
 
 #ifdef WITH_LMDB
 #include <lmdb.h>
-#include <semaphore.h>
 #endif  // WITH_LMDB
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <fcntl.h>
-#include <pthread.h>
 
 #include "modsecurity/variable_value.h"
 #include "modsecurity/collection/collection.h"