MXS-2067: Replace most SPINLOCKs

Replaced SPINLOCK with std::mutex where possible, leaving out the more
complex cases. The big offenders remaining are the binlogrouter and the
gateway.cc OpenSSL locks.

server/modules/routing/avrorouter/avro_client.cc

@@ -749,7 +749,6 @@ AvroSession::AvroSession(Avro* instance, MXS_SESSION* session)
     : dcb(session->client_dcb)
     , state(AVRO_CLIENT_UNREGISTERED)
     , format(AVRO_FORMAT_UNDEFINED)
-    , catch_lock(SPINLOCK_INIT)
     , router(instance)
     , file_handle(NULL)
     , last_sent_pos(0)

server/modules/routing/avrorouter/avrorouter.hh

@@ -153,7 +153,6 @@ public:
     int                   state;        /*< The state of this client */
     enum avro_data_format format;       /*< Stream JSON or Avro data */
     std::string           uuid;         /*< Client UUID */
-    SPINLOCK              catch_lock;   /*< Event catchup lock */
     Avro*                 router;       /*< Pointer to the owning router */
     MAXAVRO_FILE*         file_handle;  /*< Current open file handle */
     uint64_t              last_sent_pos;/*< The last record we sent */