zoo_aremove_watches segfault: zk_hashtable needs locking!

We're encountering the following segfault (stack abridged).

This happens because the watcher hashtable has no locking, and is accessed concurrently from multiple threads:

• the thread doing zoo_aremove_watches, and
• the IO thread adding / firing watchers

We encountered this with zookeeper 3.5.8, but by code inspection the code appears the same in 3.6.

 *** Aborted at 1594473472 (Unix time, try 'date -d @1594473472') ***

• • • Signal 11 (SIGSEGV) (0xae000000aa) received by PID 199 (pthread TID 0x7f1d64667700) (linux TID 1273) (code: address not mapped to object), stack trace: ***
      @ 00007f1d98dfc8b3 folly::symbolizer::(anonymous namespace)::signalHandler(int, siginfo_t*, void*)
      /src/folly/folly/experimental/symbolizer/SignalHandler.cpp:431
      @ 00007f1d95e6c89f (unknown)
      @ 00007f1d8f73de1e containsWatcher.part.3
      /src/zookeeper/zookeeper-client/zookeeper-client-c/src/zk_hashtable.c:152
      @ 00007f1d8f73e806 pathHasWatcher
      /src/zookeeper/zookeeper-client/zookeeper-client-c/src/zk_hashtable.c:456
      @ 00007f1d8f7382dd aremove_watches
      /src/zookeeper/zookeeper-client/zookeeper-client-c/src/zookeeper.c:4260
      @ 00007f1d8f738f82 zoo_aremove_watches
      /src/zookeeper/zookeeper-client/zookeeper-client-c/src/zookeeper.c:5131