Add support for predicate pushdown

[etschannen]

Currently, a lot of simple queries can only be implemented in very inefficient ways because there is no ability to filter or aggregate results on the server processes.

For instance, if you want to count the number of keys in a range, currently a client would have to read all the the data in that range to calculate that count.

One challenge with adding this feature is determining how the server processes will understand and parse user data. However, even a very limited method of parsing to begin with could provide a lot of value.