PARQUET-11: Reduce memory pressure when reading footers

based on https://github.com/apache/incubator-parquet-format/pull/2

Author: julien <julien@twitter.com>
Author: Dmitriy Ryaboy <dvryaboy@gmail.com>

Closes #7 from julienledem/reduce_metadata_memory and squashes the following commits:

96ff408 [julien] Merge branch 'master' into reduce_metadata_memory
1c382cc [julien] implement delegate instead
7664919 [Dmitriy Ryaboy] intern parquet metadata strings when reading them