Port all non documentation commits from typesafe/config

[ekrich]

This issue should remain open as a reminder to periodically review commits and port the issue across. If you find a new PR/commit, please open a new issue to handle that commit. Last lightbend/config commit is 1fc2aff, lightbend/config#597 (Nov 4, 2018)

1. Skip PR number if a doc or version update that is not needed.
2. Github uses sequential numbers for both Issues and PRs.
3. Some older sequenced PRs may have been merged after that date.

First PR is

• Fixed support for keys that look like numbers longer than an int. lightbend/config#568
• Allow application.conf to override variables in reference.conf lightbend/config#619
• Add a config.strategy to enable overwrite of properties from env vars lightbend/config#620
• Performance fix: added capacity parameter in ArrayLists lightbend/config#648
• Support for huge memory units lightbend/config#663
• Fix: Avoid loading all the environment variables as configurations lightbend/config#686
• Remove unmodifiable collection wrappers in ResolveContext lightbend/config#701
• Split out a helper method for parsing only application overrides lightbend/config#708
• use correct ParseOptions object lightbend/config#709
• Remove unnecessary array lightbend/config#731
• Avoid instantiating arbitrary classes lightbend/config#758
• fix: Optionally hide rendered environment variables lightbend/config#798
  All for now.

[ekrich]

I seems there is a regression or change in behavior from 1.4.0 to 1.4.1. See lightbend/config@379a1f0