Locking contention on com.cognitect.transit.impl.Cache #38

Hello,

During a performance tuning task, we notice a high locking time on com.cognitect.transit.impl.Cache during service startup.

I tracked it down to this code snippet:

Without changing the library itself, I wrapped our write handlers here with a WriteHandlerMap class to avoid that block, getting into this code path instead:

Doing so significantly improved our locking times; in comparable scenarios, it dropped from 53 minutes of locking time on com.cognitect.transit.impl.Cache to 1 minute and 30 seconds.

I think if we can improve this locking block, maybe replacing it with a ConcurrentHashMap or something else that avoids global locking, we may be able to improve transit performance in high concurrency scenarios.