Improve handling of storage calculation request timeouts

[ksauzz]

riak_cs_storage_d doesn't handle request timeouts well like #738. here?

crash.log

2013-11-13 14:58:51 =ERROR REPORT==== 
** State machine riak_cs_storage_d terminating 
** Last event in was continue 
** When State == calculating 
** Data == {state,[],undefined,{{2013,11,13},{5,29,58}},undefined,<0.7057.679>,{{2013,11,13},{5,29,58}},2,0,[<<"xxxxxxxxxxxxxxx-xxxxxx">>,<<"xxxxxxxxxxxxxxxxxxxxxxxx">>,<<"xxxxxxxxxxxxxxxxxxxxxxx">>,<<"xxxxxxxxxxxxxxxxxxxx">>,<<"xxxxxxxxxxxxxxxxxxxxxxx">>],true} 
** Reason for termination = 
** {json_encode,{bad_term,{error,{timeout,[]}}}} 
2013-11-13 14:58:51 =CRASH REPORT==== 
crasher: 
initial call: riak_cs_storage_d:init/1 
pid: <0.29342.291> 
registered_name: riak_cs_storage_d 
exception exit: {{json_encode,{bad_term,{error,{timeout,[]}}}},[{gen_fsm,terminate,7,[{file,"gen_fsm.erl"},{line,611}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,227}]}]} 
ancestors: [riak_cs_sup,<0.29198.291>] 
messages: [{28046540,{error,<<"timeout">>}},{23190669,{error,<<"timeout">>}},{62182351,{error,<<"timeout">>}},{121450601,{error,<<"timeout">>}},{94032563,{error,<<"timeout">>}},{22873166,{error,<<"timeout">>}},{7996964,{error,<<"timeout">>}},{90730175,{error,<<"timeout">>}}] 
links: [<0.29202.291>,<0.7057.679>] 
dictionary: [] 
trap_exit: false 
status: running 
heap_size: 28657 
stack_size: 24 
reductions: 21761 
neighbours: 
neighbour: [{pid,<0.7057.679>},{registered_name,[]},{initial_call,{riakc_pb_socket,init,['Argument__1']}},{current_function,{gen_server,loop,6}},{ancestors,[riak_cs_storage_d,riak_cs_sup,<0.29198.291>]},{messages,[]},{links,[<0.29342.291>,#Port<0.2475530>]},{dictionary,[]},{trap_exit,false},{status,waiting},{heap_size,377},{stack_size,9},{reductions,38522}] 
2013-11-13 14:58:51 =SUPERVISOR REPORT==== 
Supervisor: {local,riak_cs_sup} 
Context: child_terminated 
Reason: {json_encode,{bad_term,{error,{timeout,[]}}}} 
Offender: [{pid,<0.29342.291>},{name,riak_cs_storage_d},{mfargs,{riak_cs_storage_d,start_link,[]}},{restart_type,permanent},{shutdown,5000},{child_type,worker}]

refs: zd ticket # 6509

[kuenishi]

adding

{riakc, [
    {mapred_timeout, 120}
]}.

to app.config will avoid this problem. Find the best mapred timer.

[kuenishi]

See #759 for code review.