ciri: differentiate mainnet/testnet LS files and match IRI names

[thibault-martinez]

IRI seems to produce/consume LS files named mainnet.snapshot.meta and mainnet.snapshot.state while cIRI uses local_snapshot.meta and local_snapshot.state.
For easy exchange between both, we should match IRI names.

We also need to have different LS files for mainnet/testnet.