Add "LinesOfALargeExecFuture" abstraction

Summary:

• This is for D2457.
• We have an existing LinesOfALargeFile class for buffered, line-oriented processing of large input streams.
• Splits existing LinesOfALargeFile class into abstract LinesOfALarge and concrete LinesOfALargeFile.
• Changes next() algorithm to be iterative rather than recursive.
• Allows the user to specify a line delimiter character.
• Adds concrete subclass LinesOfALargeExecFuture.
• Document and cover all three classes.

Test Plan: Ran unit tests.

Reviewers: csilvers, btrahan

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2459