UDF API additions

This patch introduces the ability to specify a prepare and close
function for a UDF, as well as FunctionContext methods for maintaining
state across UDF invocations within a query. Many of the changes are
related to adding an Expr::Open() function which calls the UDF's
prepare function, if specified (it has to be called in Open() since
the LLVM module must be compiled first).

Change-Id: I581d90d03dff71f7ff5d4a6bef839ba6bc46b443
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1693
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins