Terms aggregations: make size=0 return all terms.

Terms aggregations return up to `size` terms, so up to now, the way to get all
matching terms back was to set `size` to an arbitrary high number that would be
larger than the number of unique terms.

Terms aggregators already made sure to not allocate memory based on the `size`
parameter so this commit mostly consists in making `0` an alias for the
maximum integer value in the TermsParser.

Close #4837