This commit fixes a parsing bug that was causing extra tags to be
generated when the incoming point contained escaped commas.
As an optimisation, the slice of tags associated with a point was
being pre-allocated using the number of commas in the series key as a hint to
the appropriate size. The hinting did not consider literal comma values in
the key though, and so it was possible for extra (empty) tag key and
value pairs to be part of the tags structure associated with a parsed
point.
6b350edf59