Fix corner-case bug in tracking of latest removed WAL segment during

streaming replication. We used log/seg 0/0 to indicate that no WAL segments
have been removed since startup, but 0/0 is a valid value for the very first
WAL segment after initdb. To make that disambiguous, store
(latest removed WAL segment + 1) in the global variable.

Per report from Matt Chesler, also reproduced by Greg Smith.