Add more OData expression support to $filter

[yorek]

Right now, the $filter REST option supports the native OData operations:

http://docs.oasis-open.org/odata/odata/v4.01/cs01/part1-protocol/odata-v4.01-cs01-part1-protocol.html#sec_BuiltinFilterOperations

feedback from some early adopters is emerging in the sense that we should support also some of the native OData query functions:

http://docs.oasis-open.org/odata/odata/v4.01/cs01/part1-protocol/odata-v4.01-cs01-part1-protocol.html#sec_BuiltinQueryFunctions

we should see how many of those can be pushed to the database engine to make sure we can push as much as possible the computation effort to the database