Add lz4_flex block format as store compressor

[PSeitz]

The lz4 block format should be sufficient for tantivy, due to the block size of 16_384 bytes.
lz4_flex is pure rust so this will improve compile times.

[fulmicoton]

In addition, it will enable to make lz4 the default. Right now, snappy is the default because I do not want to rely on a C++ third party library.