DatabaseWriter: Database Connections per Thread, Transactions, Cleanup

• make individual database connections per thread as suggested by https://libpqxx.readthedocs.io/en/7.3.0/a01348.html
• implement waive_sequence_requirement to wwirte directly instead of single-threaded buffered. This gives a massive performance boost:

With waive_sequence_requirement = false

|21:31:23.099|  (STATUS) Welcome to Allpix^2 v2.0.0+246^g4b0a00fc
|21:31:23.099|  (STATUS) Initialized PRNG with configured seed 0
|21:31:23.099|  (STATUS) Initialized core PRNG with configured seed 0
|21:31:23.386|  (STATUS) Loaded 7 modules                     
|21:31:26.092|  (STATUS) Initialized 27 module instantiations  
|21:31:26.092|  (STATUS) Multithreading enabled, processing events in parallel on 7 worker threads
|21:31:26.092|  (STATUS) Allocating a total of 3584 event slots for buffered modules
|21:31:26.097|  (STATUS) Starting event loop
|21:32:33.964|  (STATUS) Finished run of 1000 events               
|21:32:33.980|  (STATUS) [F:DatabaseWriter] Wrote 52945 objects from 18998 messages to database
                                            
|21:32:33.983|  (STATUS) Finalization completed
|21:32:33.983|  (STATUS) Executed 27 instantiations in 1 minutes 11 seconds, spending 88% of time in slowest instantiation DatabaseWriter
|21:32:33.983|  (STATUS) Average processing time is 71 ms/event, event generation at 14 Hz
|21:32:33.983|  (STATUS) This corresponds to a processing time of 497 ms/event per worker

With waive_sequence_requirement = true

|21:31:06.991|  (STATUS) Welcome to Allpix^2 v2.0.0+246^g4b0a00fc
|21:31:06.991|  (STATUS) Initialized PRNG with configured seed 0
|21:31:06.991|  (STATUS) Initialized core PRNG with configured seed 0
|21:31:07.183|  (STATUS) Loaded 7 modules                     
|21:31:09.874|  (STATUS) Initialized 27 module instantiations  
|21:31:09.874|  (STATUS) Multithreading enabled, processing events in parallel on 7 worker threads
|21:31:09.874|  (STATUS) Allocating a total of 3584 event slots for buffered modules
|21:31:09.879|  (STATUS) Starting event loop
|21:31:19.961|  (STATUS) Finished run of 1000 events              
|21:31:19.976|  (STATUS) [F:DatabaseWriter] Wrote 52945 objects from 18998 messages to database
                                            
|21:31:19.980|  (STATUS) Finalization completed
|21:31:19.980|  (STATUS) Executed 27 instantiations in 13 seconds, spending 86% of time in slowest instantiation DatabaseWriter
|21:31:19.981|  (STATUS) Average processing time is 13 ms/event, event generation at 78 Hz
|21:31:19.981|  (STATUS) This corresponds to a processing time of 91 ms/event per worker

This fixes #209 (closed)

(cc) @schioppa