Speed improvement

After profiling the syching procedure I found out that the bottleneck was not the foreign_id to local id relation but the ModelSql.create method which only allows to create single datasets. I added a create_many method to the Synchable class to create datasets with prepared statements which improved the speed very well. The RAM consumption is now even less than before.

Now I  can synch with another Tryton server over the Internet 2400 product.product datasets with all needed dependencies in kind of exactly one minute.

Now the bottleneck has moved to the fetching of records from the master server, but there are lots of possibilities for further optimizations.

The next days I will clean up the code and push it to a public repository for reviewing.