Reduce log spam in libhdfs
* Multi-threaded read support for HDFS files. This required some changes
that bubbled up to be visible outside of GenericFile:
  * Read loops in GenomeIndex.cpp and HashTable.cpp disabled (by making max read size very long)
  * GenericFile::advance() changed to take 'long long' argument instead of 'long'

* Added new 'NWaiter' utility class that lets a thread wait for n signals before continuing
Merge branch 'dev' of https://github.com/amplab/snap into dev

Conflicts:
	SNAPLib/GenomeIndex.cpp