common: fix #1818, secondary datadir paths to fall back to

p2p/discover: fix race involving the seed node iterator

The strict matching can get in the way of protocol upgrades.

nodeDB.querySeeds was not safe for concurrent use but could be called
concurrenty on multiple goroutines in the following case:

- the table was empty
- a timed refresh started
- a lookup was started and initiated refresh

These conditions are unlikely to coincide during normal use, but are
much more likely to occur all at once when the user's machine just woke
from sleep. The root cause of the issue is that querySeeds reused the
same leveldb iterator until it was exhausted.

This commit moves the refresh scheduling logic into its own goroutine
(so only one refresh is ever active) and changes querySeeds to not use
a persistent iterator. The seed node selection is now more random and
ignores nodes that have not been contacted in the last 5 days.

xeth: fixed nil pointer of filter retrieval

core: fix a formatting loop in BadHashError

This fix addresses an issue with filters that were (possibly) not yet
added to the filter queues but were expected. I've added additional nil
checks making sure it doesn't crash and swapped the installation of the
filter around so it's installed before use.

Closes #1665

eth/downloader: always send termination wakes, clean leftover

core: Add BadHashErr and test for BadHashes handling

VERSION: added version

rpc/api: don't crash for unknown blocks

tests: add test for StateTests/stCallCodes.json

Most eth RPC calls that work with blocks crashed when the block was not
found because they called Hash on a nil block. This is a regression
introduced in cdc2662c (#1779).

While here, remove the insane conversions in get*CountBy*. There is no
need to construct a complete BlockRes and converting
int->int64->*big.Int->[]byte->hexnum->string to format the length of a
slice as hex.

The test genesis block was not written properly, block insertion failed
immediately.

While here, fix the panic when shutting down "geth blocktest" with
Ctrl+C. The signal handler is now installed automatically, causing
ethereum.Stop to crash because everything is already stopped.

makefile: built in cross compilation targets

core, xeth: chain reorg move missing transactions to transaction pool

Added a `Difference` method to `types.Transactions` which sets the
receiver to the difference of a to b (NOTE: not a **and** b).

Transaction pool subscribes to RemovedTransactionEvent adding back to
those potential missing from the chain.

When a chain re-org occurs remove any transactions that were removed
from the canonical chain during the re-org as well as the receipts that
were generated in the process.

Closes #1746

Changed the version number of geth to 1.2.0 so that dev builds are now properly build (instead of master). Note to self; increase version number to 1.2.1 for our next actual release.

core: separate and contain POW verifier, extensive tests

tests: update common test wrappers and test files

core: transaction nonce recovery

When the transaction state recovery kicked in it assigned the last
(incorrect) nonce to the pending state which caused transactions with
the same nonce to occur.

Added test for nonce recovery

core: allow modifying test-chain block times

common: Update README.md for the current package name

cmd/geth: extradata is correcly initialized with console

timer bugfix when clearInterval was called from within the callback

core, eth, miner: use pure header validation