Add locking support

[alexmreis]

Implemented locking mechanism to prevent simultaneous updates from corrupting index. It is used optionally with the -l option.

Previously if 2 users were to update the repository in S3 at the same time, one of the packages would end up missing from Packages.gz and Release.

I've tried to follow the existing code as closely as possible, and added MiniTest specs for the extra code.

[alexmreis]

BTW eventual consistency is not a problem unless you use the US Standard region. See http://shlomoswidler.com/2009/12/read-after-write-consistency-in-amazon.html

[jjshoe]

@krobertson - Any chance of releasing a new gem with this option included?

[jjshoe]

Other item worth noting, README.md would need to be updated at that time showing the -l option. Happy to do a PR for that work if it'll make you more willing to release a new gem.

[jjshoe]

@krobertson - Scratch that, hold off on doing a release. Found a pretty critical bug in locking. Will make an issue shortly, and tag @alexmreis on it.

[alexmreis]

Curious as to what the issue is, we've been using the patched version in a
CI environment since last October without any major issues.

On Wed, Feb 10, 2016 at 5:35 PM, jjshoe notifications@github.com wrote:

@krobertson https://github.com/krobertson - Scratch that, hold off on
doing a release. Found a pretty critical bug in locking. Will make an issue
shortly, and tag @alexmreis https://github.com/alexmreis on it.

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Reply to this email directly or view it on GitHub
#89 (comment).

[jjshoe]

#97