Decimal: switch out the boost 128_t int with the c++ standard one.

The c++ standard int128_t is exactly what we want. It is 16 bytes, stored as 2's
complement little endian (the exact extension of the native int types). It out
performs the boost library we were using (see benchmark) and looking at the assembly
for some of the operators, I doubt we can do better. This also seems like the kind
of thing hardware might be able to do natively in the future if we stuck with the
standard implementation.

This requires minimal changes to the rest of our code so the multi int library is
abstracted away.

The standard only added int128 and not 96 or any others. We still will need to use
the boost library for some cases but nothing in the hot path. We might want to revisit
implementing an int96 in the future that is of the same format to get some space
and efficiency savings but I think we can live with just int128 for a while.

Change-Id: I137ef7be812675036dd9b6e5b48dfc5c7aa9ab37
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2200
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins