Initialize pluginManager without any tracks loaded at first, and then load rest of config secondarily

[cmdcolin]

this could open the door to allowing plugins the ability to pre-process an entire config

This might help with use cases such as the genomes.jbrowse.org jb2hubs (example motivation related to bulk loading the entirety of hg38 config via synteny lazy loading bloating the session size and leading to failure to share sessions)

[cmdcolin]

There might also be a connection to #4988 regarding lazy loading data from connections