Configurable max token length

[bratseth]

If clients feed very long tokens, or long text in fields without tokenization (match:exact/word), Vespa will produce an index containing them which is wasteful and possibly damaging.

• Add a max token length configuration.
• Set a reasonable default.

[geirst]

This is available in Vespa 8.342.10.

• The default max-token-length is 1000 (characters). Longer tokens are silently ignored, and will not be part of the index.
• Configurable per field: https://docs.vespa.ai/en/reference/schema-reference.html#max-token-length.