Additional RPCs for dapps accounts management

[tomusdrw]

Closes #3771

By default eth_accounts will return all accounts (as it was before), but it's possible to change policy for new dapps to return only specified whitelist of accounts.

Additionally recent dapps are available in RPC to simplify GUI for that.

[coveralls]

Coverage increased (+0.03%) to 85.635% when pulling 4696d7f on dapps-accounts-rpc into 6eb63a7 on master.

[gavofyork]

it's possible to change policy

policy should be secure by default.

that users are accustomed to insecure operation is no reason to continue that folly.

[tomusdrw]

Changed the default policy to return empty list as requested. Marking it as B1-relnotes since if it somehow gets released without a proper UI it makes all the dapps useless without an additional RPC call.

[coveralls]

Coverage increased (+0.02%) to 85.626% when pulling 627b8a8 on dapps-accounts-rpc into 6eb63a7 on master.

[gavofyork]

right - will leave it here until @jacogr / @ngotchac make such a UI :-)

[jacogr]

@gavofyork & @tomusdrw - UI in progress.

First round will only be to set the default accounts (available to all dapps) until we have a better way of determining the appids. (Currently it would be the contentHash, which means that each update will force the user to go re-auth their dapps.)

[tomusdrw]

I would rather merge this PR if possible even before the UI - it's still an improvement over what we have in master (it allows you to customize the policy).

[gavofyork]

ok - if so then we'll need to switch the default policy :-)

[coveralls]

Changes Unknown when pulling 1ea521e on dapps-accounts-rpc into ** on master**.