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:
```go
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:
```go
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