Change argument type of pq_sendbytes from char * to void *

Change argument type of pq_sendbytes from char * to void *

This is a follow-up to 1f605b82ba66ece8b421b10d41094dd2e3c0c48b.  It
allows getting rid of further casts at call sites.

Reviewed-by: Corey Huinker <[email protected]>
Discussion: https://www.postgresql.org/message-id/783a4edb-84f9-6df2-7470-2ef5ccc6607a@enterprisedb.com