Thrift: Generate structural fingerprints for thrift structs.

Thrift: Generate structural fingerprints for thrift structs.

Summary:
We are going to write a dense protocol soon that eliminates some metadata.
To prevent version conflicts, we want each structure to have a
structural fingerprint that will change whenever the struct changes
in a way that will affect the dense protocol.
This change computes those fingerprints and puts them in
the generated C++ code.

Reviewed By: aditya, mcslee

Test Plan:
Recompiled thrift.
Thrifted DebugProtoTest with old and new compilers.
Compared output.
Also ran thrift with those "cout"s uncommented,
examined the fingerprint material,
and verified the hashes.

Revert Plan: ok

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665227 13f79535-47bb-0310-9956-ffa450edef68