Make parquet scanner MAX_PAGE_HEADER_SIZE configurable

Parquet file page headers can get arbitrarily large because of page statistics. For example, the statistics will store the min/max value of string columns, which might be very long strings.
Currently Impala hard coded MAX_PAGE_HEADER_SIZE to 8M so it cannot access parquet file that has large page headers.
We should make MAX_PAGE_HEADER_SIZE configurable. but we should also be careful and make sure a corrupt file could not cause Impala to try to buffer the entire file in memory.