Add back tracking of forwarded ports to avoid race condition

[nkubala]

This code adds tracking of which ports we've forwarded in the port forwarder. The motivation for this is to avoid a race condition where skaffold searches for open ports for two services in a very short period of time: kubectl doesn't have enough time to forward either of the ports, so skaffold thinks the provided port is available for both services, and the first one wins while the second one is not forwarded correctly.

This also adds the pod name and the namespace to the key we're using the track forwardedPods, which fixes a potential latent issue with two containers that share the same name.

cc @briandealwis

[codecov-io]

Codecov Report

Merging #1780 into master will decrease coverage by 0.94%.
The diff coverage is 34.78%.

@@            Coverage Diff             @@
##           master    #1780      +/-   ##
==========================================
- Coverage   47.06%   46.12%   -0.95%     
==========================================
  Files         130      133       +3     
  Lines        6261     6418     +157     
==========================================
+ Hits         2947     2960      +13     
- Misses       3012     3154     +142     
- Partials      302      304       +2

Impacted Files	Coverage Δ
pkg/skaffold/util/port.go	0% <0%> (ø)	⬆️
pkg/skaffold/kubernetes/port_forward.go	41.9% <100%> (+0.55%)	⬆️
pkg/skaffold/plugin/builders/docker/docker.go	0% <0%> (ø)
pkg/skaffold/plugin/builders/docker/local.go	0% <0%> (ø)
pkg/skaffold/plugin/builders/docker/builder.go	14.58% <0%> (ø)
pkg/skaffold/build/local/docker.go	63.82% <0%> (+4.3%)	⬆️

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[priyawadhwa]

LGTM