Add support for SHOW PARTITIONS <table name>

This statement returns info on all partitions for the given table. It is implemented as
an alias for SHOW TABLE STATS, with some extended analysis checks (such as throwing if
the statement targets an unpartitioned table).

Change-Id: I19154a9d90314de18f86ba355aa5dbed808f147f
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2145
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Lenni Kuff <lskuff@cloudera.com>