Gracefully handle testrpc -f mode

[danfinlay]

Reported by @Schaeff:

When he uses the new testrpc -f $RPC_URL flag to fork an existing blockchain with testrpc, he gets some strange behavior on metamask:

• Account balance drops to 0, does not update until first block is mined.
• Tx history persists, and new txs here will be saved even after he un-forks.

These are kindof rare cases, but if developers make -f a common flag, we might want to accommodate it.

[danfinlay]

Thanks to @Schaeff for coming up with this workaround:

He says:
testrpc -f $RPC_URL -i 898989898989 solves the problem of MM merging forked and original transaction histories
(-i sets a custom networkId)
Still requires mining a block though

[kumavis]

testrpc networkId issue https://github.com/ethereumjs/testrpc/issues/189

[kumavis]

dan sez:

• the account balance issue is fixed after the first block i mined.
• the tx history is based on the network id, i think testrpc should use a unique test network id when forking

[kumavis]

TODO: confirm that when using a forked chain, initial balance RPC requests return a 0 balance

[kumavis]

fork network id https://github.com/ethereumjs/testrpc/issues/195

[danfinlay]

Closing since we have a good workaround, and have proposed better fixes in other projects.