Relock unlocked accounts after first use

[gavofyork]

No description provided.

[gavofyork]

Closes #1086

[arkpar]

Won't this break compatibility with DApps that unlock an account and do a series of transactions?
Also, it is still unsafe. An spamming attacker might eventually put their transaction in front.

[gavofyork]

i know of no dapps that do transaction batches which assume the account will remain unlocked throughout. in any case it would be a generally broken assumption, since unlockAccount is undocumented geth-specific API which makes no guarantees about how long the account should stay unlocked.

and though not a perfect solution, it's infinitely safer than leaving it as the status quo which leaves it unlocked for 20 minutes.