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
NEXUS-6882: Max Pool size set to 100

As per phone, using default OrientDB value,
still, it's exposed via properties so one can
tune it if needed.

If pool size increased, instance heap would need to
be closely monitored, and increase it if needed.
Merge pull request #12 from sonatype/nexus-2.10.x-NEXUS-6882-pool-depletion

NEXUS-6882: Orient DB pool depletion