[split] finagle-core: Log exceptions caught by ChannelStatsHandler

Problem

ChannelStatsHandler does not log exceptions caught by the Netty
pipeline. In such cases, stats are recorded for the exception name,
but no context is provided in logs to aid debugging.

Solution

Log to WARNING in `ChannelStatsHandler.exceptionCaught`.

RB_ID=392671
[split] util: Satisfy promise on fatal exception in FuturePool task

Problem

When fatal exceptions are thrown during the invocation of tasks in
ExecutorServiceFuturePool, the corresponding application-level Promise
is left unsatisfied. This results in pool-thread death and
applications failing silently.

Solution

Add a `catch` block to task-invocation in
`ExecutorServiceFuturePool.apply` which updates the result promise.

RB_ID=400033
TBR=true