address encodeint feedbacks

Summary:
(1) remove the terminator for int zero;
(2) change the decode functions to return the remaining byte slice;
(3) change the encode functions to appende to an existing byte slice;

Test Plan: go test

Reviewers: spencerkimball, andybons, petermattis

Reviewed By: petermattis

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D137