browser: save reference to istanbul by `port` rather than `"istanbul"`
browser: save reference to istanbul by `port` rather than `"istanbul"`

By using `port` to hold a reference to running istanbul servers, we can
control them independently.

dmcassel/marklogic-samplestack,supriyantomaftuh/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,supriyantomaftuh/marklogic-samplestack,supriyantomaftuh/marklogic-samplestack,laurelnaiad/marklogic-samplestack,marklogic/marklogic-samplestack,omkarudipi/marklogic-samplestack,laurelnaiad/marklogic-samplestack,laurelnaiad/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,marklogic/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,dmcassel/marklogic-samplestack,laurelnaiad/marklogic-samplestack,ryanjdew/discovery-app-w-samplestack,marklogic/marklogic-samplestack,dmcassel/marklogic-samplestack,omkarudipi/marklogic-samplestack,omkarudipi/marklogic-samplestack,dmcassel/marklogic-samplestack,supriyantomaftuh/marklogic-samplestack,omkarudipi/marklogic-samplestack,marklogic/marklogic-samplestack