* added highly experimental startup option `--query.parallelize-gather-writes`
Turning this option on will enable the `parallelize-gather` AQL
optimizer rule for certain write queries. This option is `false`
by default.
The feature is highly experimental and should not be used on a
production system. The option is therefore also hidden.
* parallelize certain write operations in AQL
* remove unneeded _vocbase
* remove unneeded _vocbase
* added startup option and tests