Improve SQLite plugin performance by using transactions

[Martin-Idel-SI]

The default storage plugin using an SQLite database currently writes every message to the database immediately. This has the benefit that on errors no messages are lost, but it incurs a performance overhead.

It would probably be best to use transactions and buffer messages, writing several at once. Since this behaviour is specific to the plugin, it might be best to implement a config for the storage plugin which can be edited by the user providing all parameters to tweak performance and memory usage.

[Karsten1987]

closing this as a potential duplicate of #123 which has some more info it. @Martin-Idel-SI feel free to re-open this in case you consider it two separate problems.