netsync: Split blockmanager into separate package.

[davecgh]

This requires #2498 and #2499.

This does the minimum work necessary to refactor the block manager into its own internal package named netsync. The main motivation is that separating this code into its own package will improve its testability and more cleanly split the details related to syncing with the network from consensus validation.

The API will certainly need some additional cleanup and changes to make it more usable outside of the specific circumstances it currently supports, however it is better to do that in future commits in order to keep the changeset as small as possible during this refactor.

It has been carefully crafted into a series of individual commits that make each step easier to review as it would be a giant blob of changes that would be very hard to review otherwise. As a case in point, a temporary interface is introduced and then removed later to allow renaming to happen in a separate commit while ensuring every individual commit continues to build and work properly along the way.

Overview of the major changes:

• Rename server blockManager field to syncManager
• Rename BMGR logging subsystem to SYNC
• Update logging to use the new subsystem
• Create the new netsync package
• Add package logger
• Tie new package logger to SYNC subsystem logger
• Move blockmanager.go -> internal/netsync/manager.go
• Rename blockManagerConfig to Config (now netsync.Config)
• Move peerNotifier to netsync/interface.go (now netsync.PeerNotifier)
• Rename newBlockManager to New (now netsync.New)
• Rename blockManager to SyncManager (now netsync.SyncManager)
• Update all references to the block manager to use the package
• Add a skeleton README.md
• Add a skeleton doc.go

This is work towards #1145.

[matheusd]

Tested all commits with run_tests.go

[dnldd]

Should be good to go after a rebase.

[rstaudt2]

Looks great. The new code is definitely clearer in terms of what SyncManager handles.

One small note, I noticed that there are still a few leftover comments referencing blockManager in internal/netsync/manager.go and some comments in server.go referring to blockmanager as well.

[davecgh]

@rstaudt2 Thanks for pointing out the missed comments. Looks like I missed some in the shuffle when breaking this all up into more easily digestible commits for review. I've modified the respective commits to update all of the relevant comments and log messages.

Note that I did not modify any mining since I noticed that it still has what appears to be leftover cruft, in the form of the unexported blockManagerFacade, from when it was split into an internal package. I'll take care of cleaning that bit up in another PR.