Investigate and address crash in erldns_zone_recource_handler most probably caused by ZoneSyncCheckWorker

[m0rcq]

On erldnsimple start:

2020-09-22 15:05:04 =ERROR REPORT====
Ranch listener erldns_admin, connection process <0.11366.0>, stream 1 had its request process <0.11371.0> exit with reason {badmap,[]} and stacktrace [{cowboy_req,reply,4,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_req.erl"},{line,785}]},{erldns_admin_zone_resource_handler,to_json,2,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/erldns_admin/src/erldns_admin_zone_resource_handler.erl"},{line,68}]},{cowboy_rest,call,3,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,1576}]},{cowboy_rest,set_resp_body,2,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,1466}]},{cowboy_rest,upgrade,4,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_rest.erl"},{line,288}]},{cowboy_stream_h,execute,3,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,296}]},{cowboy_stream_h,request_process,3,[{file,"/home/travis/build/dnsimple/erldnsimple/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,274}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,249}]}]
2020-09-22 15:11:32 =SUPERVISOR REPORT====
     Supervisor: {local,erldns_zoneclient_sup}
     Context:    child_terminated
     Reason:     normal
     Offender:   [{pid,<0.1836.0>},{id,erldns_zoneclient_fetcher_countdown},{mfargs,{erldns_zoneclient_fetcher_countdown,start_link,[]}},{restart_type,permanent},{shutdown,5000},{child_type,worker}]