In statedb:
- Store all values using prefixes for keys to allow iteration
- Add methods MTGetRoot, GetAccounts

Implement debugapi, an http server with debugging endpoints:
- debugAPI.GET("sdb/batchnum", a.handleCurrentBatch)
- debugAPI.GET("sdb/mtroot", a.handleMTRoot)
- debugAPI.GET("sdb/accounts", a.handleAccounts)
- debugAPI.GET("sdb/accounts/:Idx", a.handleAccount)

- Implement SlicePtrsToSlice and use it in all `meddler.QueryAll` sql db functions to always return []Foo instead of []*Foo

The coordinator implementation has been refactored to allow all the goroutines
to be handled from the node.

StateDB intermediate state reset when opening DB to force getting always last
Checkpoint at last BatchNum, avoiding inconsistent intermediate state.

Co-authored-by: Eduard S <eduard@iden3.io>