Add lz4 support for VABC

Add lz4 support in CowWriter/CowReader. Lz4 should offer faster
read/write performance but slightly larger COW sizes.

Download time: Reduced from 1264s to 825s (~35% faster)
Filesystem verification time: from 69s to 59s (~15% faster)
COW Size: 2.59GB to 3.21GB, (~25% regression)
Merge time: 37046ms to 27690ms (~25% faster)
Boot time: LZ4 is 2.7% faster

Overall, LZ4 offers faster read/write performance at the cost of 25%
space regression.

Test: th
Bug: 228478555
Change-Id: Ie521852fb3c9ff8a12e55b764c1eb1838f7b2181