inserttable() has an arbitrary buffer limit

[justinpryzby]

I diagnosed an issue where we were inserting (copying) a 54kb string into a column, and 14kb string into another column.
It really ought to support arbitrary-length fields, so will need to use something like realloc().

[Cito]

See also #69 for context. Currently we have a 64k fixed buffer limit, we should dynamically allocated memory instead.