event: address Feed review issues

event: clarify role of NewSubscription function

event: more Feed review fixes

* take sendLock after dropping f.mu
* add constant for number of special cases

event: fix subscribing/unsubscribing while Send is blocked

This change makes client subscriptions compatible with the new
Subscription semantics introduced in the previous commit.

This commit introduces a new Subscription type, which is synonymous with
ethereum.Subscription. It also adds a couple of utilities that make
working with Subscriptions easier. The mot complex utility is Feed, a
synchronisation device that implements broadcast subscriptions. Feed is
slightly faster than TypeMux and will replace uses of TypeMux across the
go-ethereum codebase in the future.

The Subscription type is gone, all uses are replaced by
*TypeMuxSubscription. This change is prep-work for the
introduction of the new Subscription type in a later commit.

   gorename -from '"github.com/ethereum/go-ethereum/event"::Event' -to TypeMuxEvent
   gorename -from '"github.com/ethereum/go-ethereum/event"::muxsub' -to TypeMuxSubscription
   gofmt -w -r 'Subscription -> *TypeMuxSubscription' ./event/*.go
   find . -name '*.go' -and -not -regex '\./vendor/.*' \| xargs gofmt -w -r 'event.Subscription -> *event.TypeMuxSubscription'

Commit d3b751e4 accidentally deleted a crucial 'return' statement,
leading to a crash in case of an issue with node data. This change
improves the fix in PR #3591 by removing the lock entirely.

containers/docker: update base images, add CA certs, build internally on Ubuntu

* evm: added debug flag (back)

* cmd/evm: gofmt

Gas estimation currently mostly works, but can underestimate for more funky
refunds. This is because various ops (e.g. CALL) need more gas to run than they
actually consume (e.g. 2300 stipend that is refunded if not used). With more
intricate contract interplays, it becomes almost impossible to return a proper
value to the user.

This commit swaps out the simplistic gas estimation to a binary search approach,
honing in on the correct gas use. This does mean that gas estimation needs to
rerun the transaction log(max-price) times to measure whether it fails or not,
but it's a price paid by the transaction issuer, and it should be worth it to
support proper estimates.

The Android NDK was recently removed from gomobile, leading to our Android
builds failing. Starting from https://go-review.googlesource.com/#/c/35173/ ,
gomobile requires a locally installed NDK. This PR ensures that travis installs
that too before running the build steps.

core: removal of dead-code

Removal of dead code that appeard as if we had a consensus issue. This
however is not the case as the proper error catching happens in the vm
package instead.

cmd,eth,les,internal: remove natspec support

* core: Made logging of reorgs more structured, also always log if reorg is > 63 blocks long

* core/blockchain: go fmt

* core/blockchain: Minor fixes to the reorg reporting

accounts, mobile: make account manager API a bit more uniform

common/hexutil: fix EncodeBig, Big.MarshalJSON

The code was too clever and failed to include zeros on a big.Word
boundary.

travis, appveyor, build: add source spell checking

- Use defined constants instead of hard-coding their integer value.
- Allocate secp256k1 structs on the C stack instead of converting []byte
- Remove dead code

tests: unskip test