Library: Wait for 5 /status failures before running Server actions

E.g., loading large buffers could cause the server to miss responding
to one or two /status messages. Previously, sclang would panic and act
as though the server quit for < 1 sec. Now we require 5 consecutive
missed replies before executing server quit actions. Among other things,
this prevents all SynthDefs from being re-sent (which is only necessary
if the server really died).

A manual quit of the server, via .quit, is unaffected: ServerQuit
runs immediately in this case.
class library: make number of pings before server considered dead configurable
Merge pull request #842 from miguel-negrao/serverPings

server: wait for a configurable number of pings before the server is considered dead