support basic core affinities in LocalManager and SSHManager

[amitmurthy]

Allows pinning workers to cores. Linux only feature, requires the taskset command to be installed on the system.

Closes #10645

[amitmurthy]

This branch has been tested on julia.mit.edu with the same results as #10427 (comment) .

Some things to be noted:

• It is currently a linux only feature - which is fine I guess, since most of the larger HPC type machines where this feature is required are linux.
• The current PR automatically associates julia workers with cores. affinity=:compact associates cores serially, while affinity=:balanced spreads out the julia workers across available cores.
• It is a basic feature, in the sense that it is not aware of the exact topology like that provided by https://github.com/JuliaParallel/hwloc.jl . But that could be provided by a custom cluster manager that uses hwloc.jl for more intelligent core-julia process mapping taking into account processor sockets, L1/L2 caches, etc.

I would like to merge this in a few days if there are no specific concerns.

[ViralBShah]

LGTM.

[amitmurthy]

I have changed my mind, and on further thinking, I am closing this PR in favor of adding this functionality to ClusterManagers.jl in a new LocalAffinityManager. Mainly because LocalAffinityManager can be more easily developed outside of Base to work with L1/L2 cache information, cores to socket mapping, etc using hwloc.jl.