This updates the script processor to keep a simple cache of Javascript VM
sessions instead of relying on sync.Pool for this task. The size of this
cache can be controlled by the new `max_cached_sessions` option.

The reasoning behind this change is to avoid sync.Pool discarding and
re-allocating new sessions every garbage collection cycle. For large
Javascript pipelines, allocating a new session is a very expensive
operation that can take hundreds of milliseconds or even seconds to
complete. This has a severe impact in ingestion rates.

(cherry picked from commit 7930f9e)