Make Java builds reproducible to reduce Lambda churn

[patricksanders]

When deploying psoxy to AWS using the provided Terraform, the function code bundles get updated with every plan/apply. I believe this can be avoided by making builds reproducible. This looks fairly straightforward, assuming you can require versions of dependencies that support reproducible builds.

[eschultink]

thanks @patricksanders - definitely something we'd like to deal with. Gave it a quick shot and found that a Maven plugin we're using (to "flatten" the pom.xml) isn't compatible with the approach you link for reproducible builds.

We need to investigate if we can avoid using the flatten plugin, which may avoid this.

[eschultink]

hey @patricksanders - didn't actually do this, but #226 adds simple check for the build artifact and avoids rebuilding it if it exists, unless you set force_bundle = true in your terraform.tfvars. Forcing the bundle should really only be needed if you're changing Java code, which for customer use cases we wouldn't expect.