Create DB in memory

[fsimonis]

This PR changes the merge command by first using an in-memory DB to create the database and writing it to disk once it is complete.
This leads to an 7-10% improvement and doesn't leave behind incomplete databases when the merge scripts errors.

Example using a run of 4592 files:

Creating directly on-disk (previous)

Time (mean ± σ):     10.178 s ±  0.123 s    [User: 9.315 s, System: 0.848 s]
Range (min … max):   10.030 s … 10.402 s    10 runs

Creating in-memory first (this PR)

Time (mean ± σ):      9.260 s ±  0.197 s    [User: 8.863 s, System: 0.390 s]
Range (min … max):    9.021 s …  9.537 s    10 runs

Example using a run of 15k files:

Creating directly on-disk (previous)

Time (mean ± σ):     10.858 s ±  0.104 s    [User: 9.712 s, System: 1.131 s]
Range (min … max):   10.656 s … 11.022 s    10 runs

Creating in-memory first (this PR)

Time (mean ± σ):     10.046 s ±  0.160 s    [User: 9.425 s, System: 0.611 s]
Range (min … max):    9.739 s … 10.346 s    10 runs