server: Use local addr var in version handler.
server: Use local addr var in version handler.

This modifies the OnVersion handler for server peers to use a local
variable for the remote address of the peer in order to avoid grabbing
the mutex multiple times.

There are no functional changes.

Backported from Decred.

btcsuite/btcd,ltcsuite/ltcd,btcsuite/btcd,dajohi/btcd,ppcsuite/ppcd,davecgh/btcd,ppcsuite/ppcd,tb00/btcd,tb00/btcd,wallclockbuilder/btcd,wallclockbuilder/btcd,dajohi/btcd,davecgh/btcd,ltcsuite/ltcd,dajohi/btcd