[C++][Parquet] Statistics::num_values() is misleading

[asfimport]

num_values() in statistics seems to capture the number of encoded values. This is misleading as everyplace else in parquet num_values() really indicates all values (null and not-null, i.e. the number of levels).

We should likely remove this field, rename it or at the very least update the documentation.

CC @zeroshade

Reporter: Micah Kornfield / @emkornfield

_{Note: This issue was originally created as PARQUET-2099. Please see the migration documentation for further details.}

[asfimport]

Matthew Topol / @zeroshade:
thanks for the heads up @emkornfield, i'll keep an eye on this so that I can update the Go implementation once a decision is reached.