Do not call update if a call if done to a specific plugin through the API

[nicolargo]

For the moment, every time a call is done to the API for a specific plugin (ex: /api/4/network), then all the plugins are updated:

glances/glances/outputs/glances_restful_api.py

Line 435 in ce0fc37

self.__update__()

An optimisation can be discussed on this point but it is not straighforward because plugins can be linked (for example processlist and alert).

Related to #3027 (comment)

[burgerga]

That is indeed tricky, can you create a directected acyclic graph (DAG) of the plugin inter-dependencies? If that is the case, it's probably not that hard to write a function that determines which set of plugins needs to update for which api call