cmd, core, eth: journal local transactions to disk

[karalabe]

This PR adds a disk journal to the transaction pool to back up local transactions. Upon startup, the contents of the journal are merged into the transaction pool, each transaction (and inherently sender address) marked as local. After that on startup, and every hour (configurable via cli) the current contents of the transaction pool are written out into a replacement journal and moved in place of the old one.

During the hour between the journal regeneration, the current journal is kept open append-only, and any arriving local transaction is injected into it.

[fjl]

Code looks OK, but I can imagine this breaking if many transactions are sent. Another thing to think about is how we could make this feature work with the light client.

Maybe the journal could be moved to its own object so it can send transactions into any pool, not just core.TxPool.

[karalabe]

Why would this break anything if many transactions are sent? The journal is only ever reprocessed on startup, otherwise it's just overwritten every once in a while.

[karalabe]

@fjl @Arachnid PTAL. I've separated out the tx journal into its own type. Currently the tx pool contains the journal, but the journal itself is not aware of the pool:

• The load methods which does the initial parsing of the dump and loading it into the pool accepts a callback 😱 to AddLocal to inject anything parsed directly into the pool. The reason it doesn't return all the transactions directly is to avoid loading them into memory, if most will be stale anyway and get discarded immediately. The reason for the callback vs. an interface is because currently the journal doesn't have a notion of local txs. It just operates on whatever it's given. It's cleaner imho not to mix locals in there too.
• The rotate method requires a dump of transactions to push into the journal. This breaks the other link that the journal had to the transaction pool. This does require an extra method in inside the pool to collect the local transactions, but we'd need that either way if we want to seal the journal off from the pool.

I can look into the light pool in a followup PR. In all honesty I'd rather just use this pool, but if that's not possible, I can make any adjustments needed to get the journal in there too.