[CDH4] IMPALA-1141: HBase Planner Performance

This patch improves the performance of the planning phase of a query
querying HBase tables. It removes an unnecessary second call to compute
stats and adds a new version for estimating the row count in a table.

This patch adds an incremental version to estimate the number of rows
for a set of regions. This incremental version will start querying up to
five regions to calculate the average row size and use this value to
estimate the row count based on the size of the regions on disk. Only if
the standard deviation from the average is larger than 15% query an
additional region, it will query additional regions to calculate an
average with more confidence.

If the data is balanced it will not be necessary to retrieve data from
all regions but only from a subset. In the worst case, all regions are
queried.

Change-Id: Idcb3bea81b11cb08da6d9329ba66c86aca23e170
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4901
Reviewed-by: Martin Grund <mgrund@cloudera.com>
Tested-by: jenkins