#2 Allow update to pattern as well as to data item

[Chris Greenhalgh]

The current API allows patterns to be added and
deleted, but not updated. In contrast, data items can be
updated as well. Having a pattern update operation
would avoid redundant delete/add pseudo-events when
making incremetal changes to a pattern, e.g. that
matches on a changing range of values.

This will require new messages between client and
server, as well as API extensions and implementation in
DataDelegate.