Move definition of XLogRecoveryCtlData to xlogrecovery.h

Move definition of XLogRecoveryCtlData to xlogrecovery.h

XLogRecoveryCtlData is the structure that stores the shared-memory state
of WAL recovery, including information such as promotion requests, the
timeline ID (TLI), and the LSNs of replayed records.

This refactoring is independently useful because it allows code outside
of core to access the recovery state in live.  It will be used by an
upcoming patch that introduces a SQL function for querying this
information, that can be accessed on a standby once a consistent state
has been reached.  This only moves code around, changing nothing
functionally.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/CABPTF7W+Nody-+P9y4PNk37-QWuLpfUrEonHuEhrX+Vx9Kq+Kw@mail.gmail.com