Refactor database thread

[DL6ER]

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Refactor DB thread to run exclusively either saving queries into the DB or deleting old queries (i.e. every hour at minute 59 (e.g. 14:59) no queries will be saved to the DB, but instead, the remaining amount of data (14:58:01 - 14:59:59) will be saved at the next full minute (15:00)). In addition, we introduce mutex locks for the database handle, such that it is ensured that the database handle cannot be used more than once at the same time (that won't be needed, but an additional layer of "being sure" cannot hurt).

Fixes #97 #98

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