Hooks race condition

[dgunay]

We recently found a longstanding race condition in our test code that boiled down to the fact that zerolog reuses *Events in a sync.Pool. In one of our hooks, we store the *zerolog.Event and later read it when it had aleady been reclaimed by the pool once the event sends.

The Concurrency Safety section of the docs or the doc comments in the module itself may need an update to make this more obvious.