Remove caching of footers in ParquetInputFormat

Remove footers instance variable.  This instance variable causes issues when the same ParquetInputFormat instance is used to generate splits for different files.  More specifically, it causes problems in Hive's FetchOperator when the FetchOperator is attempting to operate over more than one partition.  This issue is described in more detail at https://groups.google.com/d/msg/parquet-dev/0aXql-3z7vE/Gn5m094V7PMJ
ah that's where it's not generic.
Seems like it would read easier if the code that used this (generic) cache just said 
```
entry = cache.getEntry(path);
if (entry == null || !entry.isCurrent()) {
  entry = // create new entry
  cache.put(entry);
}