MAX_REND_FAILURES should not be 30

From src/or/rendservice.c:

/** How many times will a hidden service operator attempt to connect to
 * a requested rendezvous point before giving up? */
#define MAX_REND_FAILURES 30
/** How many seconds should we spend trying to connect to a requested
 * rendezvous point before giving up? */
#define MAX_REND_TIMEOUT 30

MAX_REND_FAILURES is way too high. To find out how lower it should be, we will need to persuade the operators of popular hidden services to collect some statistics for us.