dcdbpusher: New PluginManager class to bundle all plugin related operations. Integration outstanding

Common rest server

See merge request !3

CollectAgent should be switched to new Rest API design as soon as possible
Minor improvements to README, Makefile and PUT /analytics/reload methods

- Fixed name of log files for collectagent
- Fixed a few compilation warnings under gcc 7
- README for dcdbpusher now correctly refers to dcdbpusher.conf files
- SensorNavigator is rebuilt after plugin reloads only if data analytics
are currently running