Limit the size of HTTP requests.

Adds a new RequestBodySizeLimiter middleware to guard against
really large HTTP requests. The default max request size is 112k
although this limit is configurable via the 'max_request_body_size'
config parameter.

Fixes LP Bug #1099025.

Change-Id: Id51be3d9a0d829d63d55a92dca61a39a17629785