Use a std::mutex to protect registered_callbacks_.

[sighingnow]

What do these changes do?

The callback data structures cannot be modified concurrently.

Related issue number

Fixes #330.

[codecov]

Codecov Report

Merging #331 (8db7d21) into main (9091175) will decrease coverage by 0.55%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
- Coverage   74.30%   73.74%   -0.56%     
==========================================
  Files          62       62              
  Lines        5156     5192      +36     
==========================================
- Hits         3831     3829       -2     
- Misses       1325     1363      +38     

Impacted Files	Coverage Δ
src/server/services/etcd_meta_service.cc	78.43% <90.00%> (+0.14%)	⬆️
src/server/services/etcd_meta_service.h	92.30% <100.00%> (-0.29%)	⬇️
src/server/async/socket_server.cc	72.10% <0.00%> (-2.60%)	⬇️
src/server/util/meta_tree.cc	80.72% <0.00%> (-1.59%)	⬇️
src/server/server/vineyard_server.cc	72.20% <0.00%> (-1.40%)	⬇️
src/server/memory/memory.cc	72.77% <0.00%> (-1.24%)	⬇️
src/server/util/etcd_launcher.cc	77.77% <0.00%> (-0.68%)	⬇️
src/server/memory/stream_store.cc	62.50% <0.00%> (-0.53%)	⬇️
src/server/services/meta_service.h	79.94% <0.00%> (-0.26%)	⬇️

[andydiwenzhu]

LGTM. Nice fix!