fts: Add comment to fts_parser_deinit() describing its API

fts: Retry indexing for tika backend in case of internal server error

fts: Extend parser's deinit function interface with retriable_err_msg_r parameter the new parameter will be set to a error string in case indexing may need retrying, so it will be logged later when decision about retry will be made.

fts: Parser move try_init arguments to a single structure parser_context.

fts: If fts-parser fails, stop indexing instead of ignoring the error and continuing. This is especially important in case there's just some temporary error.

fts: Added fts-parser support for Tika It can be enabled by setting: plugin { fts_tika = http://localhost:9998/tika/ }

fts: Added unload() method to fts-parsers to allow them to cleanup before plugin unload.

Reversed recent "short utf8" changes. Solr code needs to parse the UTF8 input explicitly anyway to encode the XML characters. And all the character checks were already done in it.

fts: Added FTS_BACKEND_FLAG_BUILD_SHORT_UTF8 to require sending only short UTF8 data to backend.

fts: Don't pass NUL bytes to FTS backend. It can confuse them.

fts: Added attachment decoding via an external script.

fts: Strip text/html mails to plaintext before sending them to FTS backend.