[Parquet] Plain encoded boolean column chunks limited to 2048 values #48

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-6189

encoding::PlainEncoder::new creates a BitWriter with 256 bytes of storage, which limits the data page size that can be used.

I suggest that in

{{impl Encoder for PlainEncoder}}

the return value of put_value is tested and the BitWriter flushed+cleared whenever it runs out of space.