Turn on -Wshorten-64-to-32 and fix all the errors

Summary:
We need to turn on -Wshorten-64-to-32 for mobile. See D1671432 (internal phabricator) for details.

This diff turns on the warning flag and fixes all the errors. There were also some interesting errors that I might call bugs, especially in plain table. Going forward, I think it makes sense to have this flag turned on and be very very careful when converting 64-bit to 32-bit variables.

Test Plan: compiles

Reviewers: ljin, rven, yhchiang, sdong

Reviewed By: yhchiang

Subscribers: bobbaldwin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28689
Fix additional -Wshorten-64-to-32 errros
Fixed GetEstimatedActiveKeys

Summary:
Fixed a bug in GetEstimatedActiveKeys which does not normalized
the sampled information correctly.

Add a test in version_builder_test.

Test Plan: version_builder_test

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28707
Fixed clang compile error in version_builder_test

Summary: Fixed clang compile error in version_builder_test

Test Plan: ./version_builder_test

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: sdong, dhruba

Differential Revision: https://reviews.facebook.net/D28731