Summary:
A couple of notes from the diff:
  - The namespace block I added at the top of table_properties_collector.cc was in reaction to an issue i was having with PutVarint64 and reusing the "val" string.  I'm not sure this is the cleanest way of doing this, but abstracting this out at least results in the correct behavior.
  - I chose "rocksdb.merge.operands" as the property name.  I am open to suggestions for better names.
  - The change to sst_dump_tool.cc seems a bit inelegant to me.  Is there a better way to do the if-else block?

Test Plan:
I added a test case in table_properties_collector_test.cc.  It adds two merge operands and checks to make sure that both of them are reflected by GetMergeOperands.  It also checks to make sure the wasPropertyPresent bool is properly set in the method.

Running both of these tests should pass:
./table_properties_collector_test
./sst_dump_test

Reviewers: IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58119