Optimise sqlite inserts

[systemed]

Changes in this PR:

• Use INSERT for adding new tiles unless we're in --merge mode
• Use prepared statements for INSERT/REPLACE (requires newer version of sqlite_modern_cpp)
• Disable journaling/rollback
• Increase page_size to 64k

This reduces the time to process Antarctica (in memory) from 2hr59 to 2hr16. Peak memory consumption is 7.3GB.

Great Britain is reduced from ~7m22 to ~7m11 with unchanged memory consumption.