Release reader lock before syncing to disk.

When disk syncing a wallet file, if the wallet is flagged dirty, the
disk syncer must grab the wallet writer lock to set dirty=false.  The
disk syncing code was being called in the end of
(*Account).RescanActiveAddresses with the reader lock held (unlocked
using a defer), which prevented the writer lock from being aquired.

This change removes the defered unlock to release the reader lock
before syncing to disk.