BUG/MINOR: compression: pass the configured level to slz

BUG/MINOR: compression: pass the configured level to slz

Since its integration in 1.6 with commit 418b8c0c4 ("MAJOR: compression:
integrate support for libslz"), the slz backend only knew levels 0 and 1,
so the init functions forced the level to 0 or 1 using !!level, and the
rate limiting code only raised the level back up to 1. Since slz 1.4-dev3
was imported in commit 32582812a ("IMPORT: slz: update slz to version
1.4-dev3"), slz supports levels 1 to 3 (higher ones behave as 3), but
tune.comp.maxlevel was still capped to 1.

Let's pass the configured level to slz's init functions, and let the rate
limiting code raise the level back up to tune.comp.maxlevel.

Note that configurations with tune.comp.maxlevel above 1 will now use the
higher slz levels, which compress better but consume more CPU.

This must be backported wherever commit 32582812a is backported.