Client Improvements

[mistic]

There are some improvements we need to do on the client:

• Make validator to not allow that flushInterval < timeout.
• Tests for system stats, blocked, and default config values.
• Verify merge order between global, global defaults and method parameters.
• Add start e stop methods to the client. The stop method should assure that all metrics in buffer are sent before client closes.
• Create a constants file.
• Remove close from prototype.
• Move some helper request functions into a request class.

[senzzzi]

"Tests for system stats" are done when branch fix-systemstats-metrics is merged.