HBase sink should allow configuring row key

Currently hbase sink (attr2hbase) looks for event's attribute with key = attrPrefix param and uses it's value as a row key for the record being inserted in HBase.

Sink should allow (through params) using either:

• event's nanos (as in initial version, see FLUME-6
• event's timestamp (see discussion in FLUME-247)
• custom key, e.g. specified as % {title}

  _%

  {nanos}