Update Firestore Quickstart to use exec:exec

[BenWhitehead]

When exec-maven-plugin runs the exec:java goal, it executes the java
program in the maven process (there currently isn't an option to fork
instead of running it in process). As a side effect of this, shutdown
hooks are not run when the program has completed running. In the case
of our Quickstart that means the daemon thread for grpc and gax do not
shutdown when the program has completed running, resulting in maven
listing all the threads that are still running and showing a stacktrace.

This change uses exec:exec and manually builds the java command to be
run, thereby forcing the program to be ran in a forked process and able
to use the normal shutdown process.

Attached are a logs showing the following

• Errors printed by maven when using exec:java
• A thread dump of the maven process when using exec:java
• A log without errors after changing to exec:exec

exec_java.log
exec_java_thread_dump.log
exec_exec.log

[lesv]

@kurtisvg @nnegrey @averikitsch @dzlier-gcp These comments on exec:exec apply to more than just Firebase. We should be looking for this during reviews (and fix other samples)

[BenWhitehead]

@lesv Is there anything else I need to do from your perspective before I can merge this? (I'll rebase it to be up to date with master).

[lesv]

@BenWhitehead No - I just wanted to call out exec:java to others. I see this being used a lot. I'm happy w/ what you are doing.

[kurtisvg]

@lesv - I'll open an issue to reflect it in the sample format guidelines.