Remove unnecessary const ref on Pulse's OMQ thread ID

The thread ID is designed to be safe and cheap to copy (it's internally
just an int, or, 4 bytes on a majority modern platforms). This member is
used in different threads (i.e. the OMQ worker threads) and reads are
atomic so it also helps with a potential race condition in certain
contexts (like the local devnet).

Author: Doy-lee

src/cryptonote_core/cryptonote_core.h

@@ -831,7 +831,7 @@ class core final {
     std::unordered_map<crypto::x25519_public_key, oxenmq::AuthLevel>& _omq_auth_level_map() {
         return m_omq_auth;
     }
-    oxenmq::TaggedThreadID const& pulse_thread_id() const { return *m_pulse_thread_id; }
+    oxenmq::TaggedThreadID pulse_thread_id() const { return *m_pulse_thread_id; }
 
     /// Service Node's storage server and lokinet version
     std::array<uint16_t, 3> ss_version;