[Swift 2.3] Add @noescape to appropriate closures, matching the Swift3 branch's behavior

[swiftlyfalling]

This PR adds some @noescape markers for closures in the Swift 2.3 branch. (Matching the default behavior of the Swift3 branch.)

This should result in:

• additional compiler optimizations / small performance gains
• no need to explicitly capture self (weakly or otherwise) in synchronous methods like .writeInTransaction
• explicit matching of the Swift3 branch's closure escaping behavior for those methods, easing forward/back migration between the Swift 2.3/Swift 3 branches of GRDB

This also should be a fully backwards-compatible change.

[groue]

Hello @swiftlyfalling! Thanks for this new contribution!

Xcode 8 doesn't generate any warning related to @NoEscape closures in the Swift2.3 branch. Does this pull request target Xcode 8.1 beta? Or can it be safely used with Xcode 8 and merged right away?

[swiftlyfalling]

@groue: This can be safely used with Xcode 8 and merged right away.

Just to clarify - this PR actually has to do with fixing the escaping behavior of the closures to match up between the branches:
-Swift 1/2 defaults to "escaping" closures (unless marked @noescape).
-Swift 3 defaults to "non-escaping" closures (unless marked by @escaping).

Because of the prior default, the Swift 2.3 (and earlier) branches had some unmarked closures that were unnecessarily defaulting to "escaping", when they could have been "non-escaping".

In the Swift 3 branch, these unmarked closures are now "non-escaping" closures. Since they always could have been, this PR explicitly fixes it in the Swift 2.3 branch.

[groue]

A big thank you, as always!

[groue]

(I'll ship version 0.81.2 shortly)

[groue]

There we go, v0.81.2 is out!