upstream unencrypted headers

[michaelkirk-signal]

If this looks good to you @charlesmchen, I'll open upstream.

I took https://github.com/signalapp/YapDatabase/tree/release/unencryptedHeaders and:

1. rebased it ( no conflicts)
2. removed 764e949 which is unrelated to unencrypted headers.

I'll investigate a bit and see if we can justify upstreaming it in a separate PR, or if we should just keep it in an extension in our own project.

[michaelkirk-signal]

Upstream PR at yapstudios#466

[charlesmchen-signal]

It'd be nice to get in 764e949 one way or another since we use it in our tests.

Note that we may need to update our tests a little anyway.

[charlesmchen-signal]

LGTM

[michaelkirk-signal]

It'd be nice to get in 764e949 one way or another since we use it in our tests.

I exposed them in the category extension local to the test.

signalapp/Signal-iOS@ae488d3#diff-23ab48e4becf5545cca1bc6bd2066d1bL16