[core] Use the lz4 tool compression levels for LZ4

[fwyzard]

This Pull request:

Changes or fixes:

Make the LZ4 compression levels follow those of the lz4 command line tool:

• level 1: fast compressor, unchanged;
• level 2: the "medium" compressor added in LZ4 1.10.0; with older LZ4 versions it stays on the fast compressor, as the lz4 tool does; use LZ4_versionNumber() to check the LZ4 version at runtime;
• level 3: HC level 3;
• levels 4-9: unchanged.

The on-disk format is unchanged, so files written with levels 2 and 3 remain readable by older ROOT versions. Users who explicitly write with LZ4 level 2 or 3 will get smaller files and slower compression.

Compressed size (in MiB and as a fraction of the input) and compression speed with R__zipMultipleAlgorithm, on a sample of 10 CMS proton-proton raw data events (13.53 MiB, compressed as a single buffer):

level	before	after, LZ4 1.10.0	after, LZ4 1.9.2
1	11.68 MiB (86.3%), 1339 MiB/s	unchanged	unchanged
2	11.68 MiB (86.3%), 1347 MiB/s	11.31 MiB (83.6%), 264 MiB/s	unchanged
3	11.68 MiB (86.3%), 1343 MiB/s	11.07 MiB (81.8%), 77 MiB/s	11.07 MiB (81.8%), 76 MiB/s
4	11.03 MiB (81.5%), 72 MiB/s	unchanged	unchanged
...	...	unchanged	unchanged
9	11.00 MiB (81.3%), 52 MiB/s	unchanged	unchanged

Without this PR the levels 2 and 3 are identical to level 1.

Checklist:

• tested changes locally
• updated the docs (if necessary)