Optimize ZFS kernel settings

Summary:
Always require prefetch: our lower RAM server is 4G, reporting 4073M,
and so disabling prefetch (as 4096M are required).

For low RAM servers, limit ZFS ARC to 2G: that allows InnoDB to use RAM too.
That allows 1G InnoDB, 2G I/O and to keep 1G for other applications.

References:

• https://www.freebsd.org/doc/handbook/zfs-term.html
• https://www.freebsd.org/doc/handbook/zfs-advanced.html

Ref T1268

Test Plan: Watch Ysul performances in November

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1268

Differential Revision: https://devcentral.nasqueron.org/D1184