Provide quick row count estimation for Ustat

Update Statistics needs an estimation of the cardinality of an HBase
table, which to this point has been provided by the result of selecting
count(*) from the table with an internal query. This incurred a
significant overhead for large files, and also occasionally resulted in
an 8448 error due to a known coprocessor problem. The approach
implemented by this fix is to access the HFiles through the FileSystem
interface and read the EntryCount field in the trailer block of each
file. Some sampling of initial data blocks is done to determine the
expected number of missing KevValues due to nulls and the number of
non-PUT KeyValues. The number of rows is estimated by dividing the
adjusted count by the number of columns in the table. The MemStore of
each of the table's regions is checked to get the total storage for
the table outside of HFiles, and the number of rows in memory is
estimated using the total MemStore size and the size-to-rowcount ratio
for the HFiles.

Change-Id: I7435ec3c765992084947b9dc7f8540c779f1f5d3
Closes-Bug: #1321857
Merge "Provide quick row count estimation for Ustat"