preallocate free space

[andrew-ld]

Start with a clear and concise description of what problem you are trying to solve.

avoid that the insertion of entities in the database fails because there is no more free space in the device.

Describe the solution you'd like allow objectbox to preallocate free space (during database creation) from device memory for further usage.

Describe alternatives you've considered I don't give a damn about devices with little free space.