Eliminate context['engine']['service']

[hasegaw]

context['engine']['service']['function_name'] has been provided for a while, but it should be eliminated.

Background

Originally I wanted to restrict access to the engine object from plugins, because the engine has several references to the important data (e.g. users' twitter/stat.ink/slack authentication ID) that should not to be exposed to unverified plugins.

However recently I realized current design doesn't work as expected, because bound method such as context['engine']['service']['function_name'] allows access to IkaEngine object, because of Python language design.

dhcp44-80:IkaLog_github14 hasegaw$ python3
>>> class hoge():
...     def a(self):
...         print('hello')
...
>>>
>>> a = hoge().a
>>> a
<bound method hoge.a of <__main__.hoge object at 0x10f5c5c50>>
>>> a.__self__
<__main__.hoge object at 0x10f5c5c50>

Therefore context['engine']['service'] is not a good, redundant approach that should be altered by ref to the engine (introduced in commit c584828) , and should be removed.