Added sentry simulator implementation

[mh0lt]

This adds a simulator object with implements the SentryServer api but takes objects from a pre-existing snapshot file.

If the snapshot is not available locally it will download and index the .seg file for the header range being asked for.

It is created as follows:

sim, err := simulator.NewSentry(ctx, "mumbai", dataDir, 1, logger)

Where the arguments are:

• ctx - a callable context where cancel will close the simulator torrent and file connections (it also has a Close method)
• chain - the name of the chain to take the snapshots from
• datadir - a directory potentially containing snapshot .seg files. If not files exist in this directory they will be downloaded
• num peers - the number of peers the simulator should create
• logger - the loger to log actions to

It can be attached to a client as follows:

simClient := direct.NewSentryClientDirect(66, sim)

At the moment only very basic functionality is implemented:

• get headers will return headers by range or hash (hash assumes a pre-downloaded .seg as it needs an index
• the header replay semantics need to be confirmed
• eth 65 and 66(+) messaging is supported
• For details see: `simulator_test.go

More advanced peer behavior (e.g. header rewriting) can be added
Bodies/Transactions handling can be added

[taratorio]

fyi we were chatting with @battlmonstr about trying to stick to some common code style, like google's code style guide

one example was to stick to unbroken lowercase imports e.g. https://google.github.io/styleguide/go/decisions#package-names

we are only starting the conversation now though (no action needed), will probably be best to introduce some linter changes later on once we have some alignment with everyone

[mh0lt]

This is not a package name its a disambiguation.

I think you get better code readability if you use this model for name disambiguation because:

• its clear its not a package
• it gives some clue as to where the package came from

I think its better than the package1, package2 etc.

[taratorio]

I agree, it is better than package1, package2 to disambiguate, however it could be sentryif or sentryinterfaces - ie avoiding snake case