* whisper: mailserver test introduced, refactoring

* whisper: validation test updated

* whisper: max number of peers fixed

* whisper: verification bug fixed

* whisper: esthetic fix

* whisper: interface changed to simplify the transition to v5

* whisper: preparation for version switch

* core,eth,internal: Added `debug_getBadBlocks()` method

When bad blocks are discovered, these are stored within geth.
An RPC-endpoint makes them availablewithin the `debug`
namespace. This feature makes it easier to discover network forks.

```

* core, api: go format + docs

* core/blockchain: Documentation, fix minor nitpick

* core: fix failing blockchain test

params: 1.5.9 stable

Dockerfile: support building USB on Alpine, ignore temp files

cmd/swarm: handle SIGTERM unix signal for clean exit

Revert "params: core, core/vm, miner: 64bit gas instructions (#3514)"

This reverts commit 8b57c494.

accounts: initial support for Ledger hardware wallets

* swarm/network: integrity on incoming known chunks
* swarm/network: fix integrity check for incoming chunks
* swarm/storage: imrpoved integrity checking on chunks
* dbstore panics on corrupt chunk entry an prompts user to run cleandb
* memstore adds logging for garbage collection
* dbstore refactor item delete. correct partial deletes in Get
* cmd/swarm: added cleandb subcommand

- Handle a data race where a Ledger drops between list and open
- Prolong Ledger tx confirmation window to 30 days from 1 minute
- Simplify Ledger chainid-signature calculation and validation
- Simplify Ledger USB APDU request chunking algorithm
- Silence keystore account cache notifications for manual actions
- Only enable self derivations if wallet open succeeds

travis: split Android off OSX, use native image

cmd/abigen: parse contract name as abi identifier

closes #3650

event: use sync.Once for init for faster/cleaner locking

Signed-off-by: NDiSiqueira <dieg0@live.com>

core/genesis: add support for setting nonce in 'alloc'

event: add new Subscription type and related utilities