Consistently name Groovy scripts with the same content

[dakrone]

When adding a script to the Groovy classloader, the script name is used
as the class identifier in the classloader. This means that in order not
to break JVM Classloader convention, that script must always be
available by that name. As a result, modifying a script with the same
content over and over causes it to be loaded with a different name (due
to the incrementing integer).

This is particularly bad when something like chef or puppet replaces the
on-disk script file with the same content over and over every time a
machine is converged.

This change makes the script name the SHA1 hash of the script itself,
meaning that replacing a script with the same text will use the same
script name.

Resolves #12212

As a test for this, I did the following:

• Configure the resource watcher to check for new scripts more frequently (every 100ms)

watcher.interval.low: 100ms
watcher.interval.medium: 100ms
watcher.interval.high: 100ms
resource.reload.interval.low: 100ms
resource.reload.interval.medium: 100ms
resource.reload.interval.high: 100ms

• Start ES with a 1.7 JVM (since 1.8 removed permgen)
• Run a script that constantly updated a script file with the same content over and over, causing it to be re-compiled by Elasticsearch:

while true; do
  echo "ctx._source.counter += 1" > config/scripts/myscript.groovy
  echo "ctx._source.counter += 2" > config/scripts/myscript.groovy
done

Without this change, permgen grows linearly, then the ES node hits OOME in permgen:

And with this change, the classes are able to be unloaded, because they share the same class name (SHA1 of the content):

[jasontedor]

Nice. LGTM.