relay: persistent transaction management

[drortirosh]

Relay should:

• Save all sent (pending) transactions.
• Remove transactions as they are mined.
• When a relay process is started, check if there are pending transaction (more than X minutes)
• re-send such pending transaction, with boosted gas price (e.g. eth.gasPrice*1.5)
  Rationale: if a transaction blocks the relay (even if it agreed for a given gas price), it blocks the relay from processing new transactions, so it's better to boost it, even if it's on its own expense.

[drortirosh]

since we're already using geth sources (for abigen), this DB should probably use leveldb..

[spalladino]

I've started working on this one, hope to have something ready by early next week.