Implement basic peer manager

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Implements a peer manager, which is basically just a threadsafe hashmap wrapped in an Arc for concurrent access across the network polling thread and the syncing thread
  • Can definitely be improved to handle the invalid responses or disconnect from the peer entirely under certain circumstances, but I decided to not get too opinionated or in depth in this PR
• Updates config file usage to make it easier and more consolidated to swap out the data directory for the db and network keypair
• Switched basically all references to the db to Arc pointers to allow these structs to be more easily moved across threads and allow concurrent access to the db. The database only requires immutable access because every implementation including in memory uses locks (idk about rocksdb iternally actually) to get mutable access

easiest way to test current usage is running:

cargo run -p forest -- -c config.toml

in multiple proccesses and changing the config file for each where the data dir config is overriden like so:

data_dir = "test_dir"

or whatever directory you want the chain data placed.

Reference issue to close (if applicable)

Closes #250

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