Do not generate static responses

[krizhanovsky]

Currently static responses like 30x or 50x responses are generated for each case with skb and pool allocations, and that's not good for performance. Rather the responses should be made from request skbs which lead to the error response.

Since we need very similar skb modifications, the issue is somewhat linked with #47.

Basically, it seems all the Web servers allow to write custom error response HTML pages, so we have to keep such pages in the Web cache and provide interface to a user to modify the pages.

[krizhanovsky]

Somewhat linked with #123.

The fix of #658 (Client friendly requests blocking) introduces static response bodies which are copied now to skbs. Meantime, we can just reuse them as skb page fragments.

[krizhanovsky]

With HTTP/2 we can not pre-generate HTTP messages: any message must be encrypted by a session key and framed according to current streams prioritization and window #1196 , so the task is only about skb reusage. skb reusage is essentially just about avoiding allocation of skb since we still need to rewrite all its fields, and this is point 5 of #391.