[bugfix]add wait_add_target_list to store the target

[dabue]

Add a wait_add_target_list to store the target before added to the target_list actually.
Using the function locking_target_list, the timer delays adding targets, so it will get an error if use the get_target_status function to get the status of a node before added.
use the wait_add_target_list to store target's default is_healthy status.

fix #43

[dabue]

At v1.2.0 , some errors got like:
[error] 46#46: 14188959 failed to run balancer_by_lua: /opt/app/test_proj/deps/share/lua/5.1/resty/healthcheck.lua:247: attempt to index field 'targets' (a nil value)
stack traceback:
/opt/app/test_proj/deps/share/lua/5.1/resty/healthcheck.lua:247: in function 'get_target'
/opt/app/test_proj/deps/share/lua/5.1/resty/healthcheck.lua:424: in function 'get_target_status'
/opt/app/test_proj/lua/ins_breaker/http/balancer.lua:334: in function 'load_balancer'
/opt/app/test_proj/lua/circuit_breaker.lua:259: in function 'http_balancer_phase'
balancer_by_lua:2: in main chunk while connecting to upstream, client: 10.23.178.7

[Tieske]

looks like the same issue as #44