IMPALA-2444: Part 4: Parquet scanner perf improvements
IMPALA-2444: Part 4: Parquet scanner perf improvements

This patch avoids two branches on needs_conversion_ in ReadValue.
The majority of column types never require conversion, so we can
avoid this branch based on template arguments in most cases.

Change-Id: I4861746fef9968881e775b33657d1356156c731b
Reviewed-on: http://gerrit.cloudera.org:8080/1112
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>

michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala