IMPALA-913: Revisit the use of FNV Hash in exchange

FNV hash has the property that the least significant bit of the hashed value
is just the XOR of the LSBs of its input bytes. This results in poor
distribution of rows when the partition keys are duplicated -- for example,
if the partition key is (l_orderkey, l_orderkey). A recommended technique
to mitigate this is to generate a larger hash and use XOR-folding to reduce
it to the desired length.

In this patch FnvHash has been modified to use generate a 64-bit hash and
fold the result down to 32-bits. It has been renamed FnvHash64to32 to make
this explicit.

Change-Id: Ie12ad3f863fca15092803d3e4d616a654cb8d244
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2101
Reviewed-by: Srinath Shankar <sshankar@cloudera.com>
Tested-by: jenkins