lib-http: Restructured the header, message, request, and response parsers to have an extensible and consistent API using flags. Extended the test suites with a few cases that test parsing with and without the STRICT flag.

lib-http: request parser: Added support for explicitly finishing payload of previous request.

lib-http: Added support for parsing Expect: header (currently only accepts `100-continue').

lib-http: http-request-parser: Added function to test whether payload from previous request is still being parsed. This is needed in the server implementation to check whether a request is completely read.

lib-http: Implemented limits on request method and target length.

lib-http: Adjusted message and request parsers to return an error code.

lib-http: Implemented limits on overall HTTP header size, size of individual header fields and the number of fields in the header.

lib-http: Unified http-request.h and http-response.h headers. Renamed struct http_response_header to struct http_header_field, encapsulated the array in struct http_header and put it all in http-header.h/c Added inline utility functions for header querying and getting response/request payload size.

lib-http: Improved message header and body parsing for better RFC compliance. Added pre-parsed transfer-encoding and connection header content (array) to parsed message struct. Fixed message body handling for when both transfer-encoding and content-length headers are missing. Now duplicates of unique important message headers yield an error.

lib-http: Adjusted message parser to accept pool from caller.

lib-http: Adjusted response and request parsers to accept a request/response object to fill with data, rather than have it return one.

lib-http: Implemented http-request-parser for HTTP servers. Moved code common with it and http-response-parser to http-message-parser.