esp_http_server: Provide apps an option to let http_server ignore sess_ctx changes

esp_http_server: Provide apps an option to let http_server ignore sess_ctx changes

By default, if a URI handler changes the http session context,
the webserver internally clears the older context after the handler
returns. However, if applications want to change this behavior and
manage the allocation/de-allocation/freeing themselves and let the
server handle only the "socket close" case, this commit provides such
an option.