enb: remove usleep after sending RRC Connection Release to UE

this cause a "dead-lock" with ZMQ because it made the stack
thread sleep for 50ms. With a sync-queue size of 1 (now the default
using ZMQ) the zmq-based radio to timeout. as a result
the rx/tx between UE and eNB stopped so the UE never reached
RRC Idle state.

srsenb/src/stack/rrc/rrc.cc

@@ -537,10 +537,9 @@ void rrc::process_release_complete(uint16_t rnti)
     if (!user_it->second->is_idle()) {
       rlc->clear_buffer(rnti);
       user_it->second->send_connection_release();
-      // There is no RRCReleaseComplete message from UE thus wait ~50 subframes for tx
-      usleep(50000);
     }
-    rem_user_thread(rnti);
+    // delay user deletion for ~50 TTI (until RRC release is sent)
+    timers->defer_callback(50, [&]() { rem_user_thread(rnti); });
   } else {
     rrc_log->error("Received ReleaseComplete for unknown rnti=0x%x\n", rnti);
   }