Fix invalid InfluxDbSink session handling

[johanneshiry]

The current implementation of the InfluxDbSinks has several architectural issues that causes a huge performance drop when executing queries on the database. Especially in a concurrent environment with several thousand write calls the existing implementation is not usable at all. Furthermore, the current implementation prevents tuning performance relevant parameters.

Hence some major refactoring is required incl. changing method signatures.

• adapt InfluxDbConnector to hold only one session per connector and not creating a new session for each call on getSession()
• provide constructors for InfluxDbConnector that allows for passing in InfluxDb session tuning parameters (especially BatchOptions)
• remove null for scenarioName inInfluxDbConnector` constructor (relates to InfluxDbSink throws NPE #247)
• shutdown session in InfluxDbConnector#shutdown()