http: lingering close (like nginx have) for entity-too-large

http: lingering close (like nginx have) for entity-too-large

By lingering close I mean something what nginx have for this name, by this term
I mean that we need to read all the body even if it's size greater then
`max_body_size`, otherwise browsers on win32 (including chrome) failed read the
http status - entity-too-large (while on linux chrome for instance are good),
and also this includes badly written http clients.

Refs: #321

v2: do this only under EVHTTP_SERVER_LINGERING_CLOSE