Replace RequestFilter with HTTPMiddleware

[silverstripe]

Replace RequestFilter with HTTPMiddleware

[robbieaverill]

I've been looking at this with @tractorcow - the reason that this RequestFilter exists is to bind the manipulation capture for the Database class.

We can replace this with Injector definitions that overload the class instead, or potentially even just remove all of the capture classes and use existing (and one new) extension points e.g. DataObject augmentWrite.

I tested this locally with mostly successful results, but the test suite took 400% longer to run which could be an indication that it's not a sustainable approach.

The Injector approach is good for now as it will improve the quality of the fulltextsearch module while also removing the need for this ticket, but it won't solve the same problem in the silverstripe-auditor module, which also needs to do the same thing. The extension approach would work for both modules, but again may be slower than normal.

---

For now I'm going to use Injector overloading instead, which makes this ticket redundant.

Closing. The changes will be implemented in #160 whilst re-implementing PostgreSQL.