Fixed slave to validate slave id during (re-)registration.

Review: https://reviews.apache.org/r/12122
Added missing os/exists.hpp to un-break the build.
Fixed a misplaced assertion in ReaperTest.TerminatedChildProcess.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/12145
Increased timeouts in replicated log tests.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/12121