removed 'const' warnings.

removed 'const' warnings.

* hash.h (hash_table_put): Make argument "value" const.
* hash.c (hash_table_put): Make argument value const.  Cast `value' to
void.
* http.c (request_set_header): Make argument `name' const.  Cast `value'
and `name' to void*.
(request_remove_header): Make argument `name' const.
* url.c (url_file_name): Make `index_filename' static.
* warc.h (warc_write_cdx_record): Make `url', `timestamp', `mime_type',
`payload_digest', `redirect_location', `warc_filename', response_uuid'
arguments const. Make `checksum' const.
* warc.c (warc_write_date_header): Make the `timestamp' argument const.
Make `extension' const.
(warc_write_cdx_record): Make `url', `timestamp', `mime_type',
`payload_digest', `redirect_location', `warc_filename', response_uuid'
arguments const. Make `checksum' const.