External Data Source: Few small API changes

* Rename getStats() to prepare()
* Adds TRowBatch.num_rows to indicate number of rows when no cols are
  materialized
* Changes api and sample poms to produce source jars

Change-Id: I02dcc89e27716978708386cfc3f7940ee5dbc023
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2406
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins
IMPALA-923: Boolean slotrefs not marked as assigned in inline views

A boolean slotref predicate that could be pushed into an inline view
would not be correctly marked as assigned, leading to an extra select
node being introduced to evaluate it. This was because the id of the
expression after substitution would change (see createInlineViewPlan()),
but only the post-substitution conjunct IDs were marked as assigned.

This bug only affected standalone slotrefs; other exprs (like casts, or
explicit predicates referencing a slotref) would not change their ID
under substitution.

Change-Id: I4127528b4aec25c966a4d186ddc98a68502b90c1
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2430
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins