td update from node + bugfix

p2p: a bunch of fixes

- reorg and simplify AddBlock
- introduce nodeCache
- TestPeerPromotionByTdOnBlock unskipped and passes
- move switchC/idleC channel creation around: solves deadlock (now respects the contract with section process: either can activate or complete at any one time)

This is supposed to apply some back pressure so Server is not accepting
more connections than it can actually handle. The current limit is 50.
This doesn't really need to be configurable, but we'll see how it
behaves in our test nodes and adjust accordingly.

As of this commit, p2p will disconnect nodes directly after the
encryption handshake if too many peer connections are active.
Errors in the protocol handshake packet are now handled more politely
by sending a disconnect packet before closing the connection.

netWrapper already sets a read deadline in ReadMsg.

There were multiple synchronization issues in the disconnect handling,
all caused by the odd special-casing of Peer.readLoop errors. Remove the
special handling of read errors and make readLoop part of the Peer
WaitGroup.

Thanks to @Gustav-Simonsson for pointing at arrows in a diagram
and playing rubber-duck.

This commit introduces a new (temporary) peer selection
strategy based on random lookups.

While we're here, also implement the TODOs in dialLoop.

xeth: fix #640, panic converting nil recipient to hex.

Fetching the recipient address from a transaction was changed to return nil
instead of a zero-address, but this code path was not updated, so whenever
a contract was created, a nil panic occured.

bugfix as a result of PR 671

@zelig: Temporarily commented out TD check untill the rest of the network has
been fixed.

Merge branch 'frontier/blockpool' of https://github.com/ethersphere/go-ethereum into ethersphere-frontier/blockpool

- add peer switch channel arg to activateChain - no peer locking within
- proper locking in AddBlock - fixes deadlock issue
- comment out TD check and skip incorrect TD test again for hotfix

- queued       bool // flag for blockpool to skip TD check
- set to true when future block queued
- in checkTD: skip check if queued
- TODO: add test (insertchain sets future block)

- TestPeerPromotionByTdOnBlock renamed and skipped for now test should pass iff
  if TD is updated based on an agreement
- senders register in AddBlock, flag records if they are coming from newblock
  message (and therefore advertise their TD with the block) or block message
  (TODO: latter are stored on the cache and updated by checkTD call;
  protocol should also call AddBlock on newblock messages by non-best peers)
- remove TD update from optional TD field in addBlock: this is no longer part of
  the eth protocol spec -> TODO: reflect in wiki
- only initialise peer map if at least two

- do not break from headsection on error
[remove peer after protocol quit will close switchC, until then head block can arrive and block on channel while keeping peers lock causing a deadlock.]
- more careful locking in AddBlock