NEXUS-6882: Orient DB pool depletion

As learned from npm sources, it uses request package that in turn
uses Agent package that builds on node's http package. None of these
override http.maxSockets, hence npm client uses the default value
(if anyone knows differently, speak up!) which is 5.

This also explains why simple "load" tests worked with 2 npm
clients but depleted npm plugin OrientDB pool when 3+ npm was
running (default pool size was too low: 10).

This change also exposes the pool size settings via nexus.properties

Note: OrientDB default pool size is 100, unsure why such low size for
maxPool was chosen initially at all.

Issue
https://issues.sonatype.org/browse/NEXUS-6882

CI
http://bamboo.s/browse/NX-NPMF18
Why 50? Is this arbitrary? What estimated load can Nexus handle with this pool size? How many concurrent installs /deploys / searches

I don't disagree with this pull request or increasing pool size - but I have no idea if it will be generally enough or how increasing it will impact Nexus stability overall. And I have no guidelines to give customers should they encounter problems.