Gracefully handle large array items

[jasonatball]

A user created a packet which contained the following item:
APPEND_ARRAY_ITEM DATA 16 UINT 16777216 "Buffer of two-byte unsigned integer pixel values"

COSMOS happily parsed this definition but then Packet Viewer chokes when trying to display it and Script Runner freezes when trying to check the data in a script. I think this is just an issue with ARRAY_ITEM but we should investigate boundary conditions with STRING and BLOCK as well.

We either need to disallow certain size items or pop up a warning saying your performance will suffer. The cut off could be difficult to determine considering different machine specs.