Added client_body_early_read directive

Added client_body_early_read directive

The directive accepts predicates.  When enabled, request body is read
immediately after the headers and before choosing a location.
Body read settings from the server block are used in this case.

Example:

http {

    map $http_content_type $is_json {
        application/json 1;
    }

    server {
        listen 8000;

        client_body_early_read $is_json;
        client_max_body_size 256;
        client_body_buffer_size 256;

        return 200 $request_body;
    }
}