Calls to mutate make start up very slow

- Version: LogStash v5.4.1
- Operating System: Ubuntu 16.04 inside a Docker 17.03-ce container on NixOS 17.03
- Config File (if you have sensitive info, please remove it): See below
- Steps to Reproduce: Start LogStash

This is likely related to #6460 and/or #6024

Note that I am providing LogStash with this `jvm.options` file:
```
-XX:+UseParNewGC
-XX:+UseConcMarkSweepGC
-XX:CMSInitiatingOccupancyFraction=75
-XX:+UseCMSInitiatingOccupancyOnly
-Dcompile.invokedynamic=true
-Xms2g
-Xmx5g
-Djava.awt.headless=true
-Dfile.encoding=UTF-8
-XX:+DisableExplicitGC
```

We have a filter configuration similar to this (which is inefficient but bear with me):
```
filter {
if "arobat.exe" == [process_name] and "Adobe Systems" in [binary][digsig_publisher] { mutate { add_field => { "whitelisted" => true } } }
if "autonomyirm.exe" == [process_name] and "Hewlett-Packard" in [binary][company_name] { mutate { add_field => { "whitelisted" => true } } }
if "ccsvchst.exe" == [process_name] and "Symantec" in [binary][company] { mutate { add_field => { "whitelisted" => true } } }
if "ccsvchst.exe" == [process_name] and "symantec" in [process][cmdline] and "symantec" in [binary][digsig_publisher] { mutate { add_field => { "whitelisted" => true } } }
if "arobat.exe" == [process_name] and "Adobe Systems" in [binary][digsig_publisher] { mutate { add_field => { "whitelisted" => true } } }
if "autonomyirm.exe" == [process_name] and "Hewlett-Packard" in [binary][company_name] { mutate { add_field => { "whitelisted" => true } } }
... # Imagine 1200 of these, all similar in structure
}
```

Startup up LogStash with this configuration takes roughly 10 minutes.
I ran a few things against the Docker container, it seems the JVM is GC'ing like crazy, lots and lots of little objects are created and garbage collected.
A single CPU core constantly sits at 100% (presumably because of the GC activity) during the 10-minute startup.

Changing the configuration to a giant disjunction like so (logically identical if we ignore short-circuiting at runtime):
```
filter {
if (
("arobat.exe" == [process_name] and "Adobe Systems" in [binary][digsig_publisher]) or
("autonomyirm.exe" == [process_name] and "Hewlett-Packard" in [binary][company_name]) or
("ccsvchst.exe" == [process_name] and "Symantec" in [binary][company]) or
... # Image 1200 of these
) {
mutate {
add_field => { "whitelisted" => "true" }
}
}
}
```

Now startup takes less than 30 seconds.

Based on these observations it seems that calls to `mutate` (and possibly plugins in general?) really slows down LogStash startup.
Given that all our `mutate` steps are identical, some sort of optimization could have happened here. Or at the very least, start up should not be taking 10 minutes.