Wrap glog in a logging utility package.

Summary: First step toward adding a global logging functionality, this replaces calls to glog with calls to an internal library.

Test Plan: Make test, this change is a noop.

Reviewers: spencerkimball

Reviewed By: spencerkimball

Subscribers: jiangmingyang, team

Differential Revision: http://phabricator.cockroachdb.org/D126