Fix data channel data races and globally shared plugin state

[saligrama]

Issue:

Currently, the data channel has several concurrent goroutines that modify shared data. When running under the Go race detector go run -race, multiple warnings are fired about data races pertaining to the data channel. Moreover, the session registry holds one live instance of each session type (shell / port), meaning that when using the plugin as a library and trying to open multiple sessions at a time, these sessions overwrite each other's data.

Description of changes:

• Add locking to the data channel and locking where shared data is modified
• Modify the session registry to store constructors of session types, rather than live session instances.

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