extern: Update lz4 to v1.10.0

This gets us ~5% faster compression which improves build/update
throughput. This version also introduces support for compression level
2, but using it regresses file sizes by ~15% (and builds them 1.5x
faster) which for now isn't a great tradeoff.

Update file data compression level to 3 (from 5)

This results in ~3% larger files but the build throughput is ~35% higher.

The original compression level has been 9; it was changed from 9 to 5 in
2017, for 1% regression in size vs 50% gain in throughput. This change
represents a more modest gain in throughput and a larger regression in size,
but the query performance is generally not impacted here either.

This should provide a little bit of a better balance on modern systems where
it takes a lot of cores for compression to beat the disk read throughput.

Add a single-line license blurb to all sources

extern: Update third-party libraries

Updated re2 to 2019-01-01
Updated lz4 to 1.8.3

Fix repository URL