Make TAU processor threadpool size configurable

The default type and unit processor threadpool size of 10 was causing a
massive bottleneck during ingestion. It would hold up the rest of the
ingestion pipeline during initial load of the metadata cache and when
persisting large loads of new entries in the metrics_metadata CF, as it
does the writes one at a time.
Set the TAU processor core pool size too
Add batched meta writes method to AstyanaxWriter
Add batched meta reads method to AstyanaxReader
Add batched interfaces for meta fetch and push in MetadatIO
Add ability to use the batched methods for meta fetch and push for metadata cache
Add instrumentation around scanMetrics and checkMeta
Log the entire exception in RollupType cacher on cache exceptions
Place holder methods in MetadataCacheIntegrationTest
Add AstyanaxMetadataIO integration test
Add test cases for batched mode for meta reads and writes
Merge pull request #308 from lakshmi-kannan/bf_batched_meta_reads_writes_merge

RFR: Batched meta reads and writes
Merge pull request #298 from dlobue/configure_type_and_unit_processor_threadpool_size

Make TAU processor threadpool size configurable