Remove numTuples in Statistics for now.
Update scaladoc for a field and move it to @param section.
Get size info from metastore for MetastoreRelation.

Additionally, remove size estimate from ParquetRelation since the Hadoop
FileSystem API calls can be expensive (e.g. S3FileSystem has a lot of
RPCs).
Add a note on performance of statistics.
Avoid calling statistics on plans if auto join conversion is disabled.