`cudf::jni::convert_table_for_return()` is usually used on tables returned from a libcudf API call.
It currently requires an l-value reference for its table argument. This necessitates parking the
result of libcudf call in an avoidable temp variable.
This commit adds the option to use an r-value reference. This allows table expressions to be used
directly, reducing clutter.

Note:
  1. The previous signature is retained, because not all call sites can use the r-value interface
     cleanly. (E.g. when the libcudf call is complex.)
  2. The third argument (vector<unique_ptr<column>>) has been converted from l-ref to r-ref,
     so that an empty default can be introduced.

This commit also includes minor code cleanup in the periphery of calls to `convert_table_for_return()`.
Call it collateral damage.

Added/Fixed jpointerArray::begin()/end() signatures.

…table-jni