p2p: throttle all discovery lookups

core, miner, xeth: renamed gas methods

core, ethdb, trie: validate database errors

Lookup calls would spin out of control when network connectivity was
lost. The throttling that was in place only took effect when the table
returned zero results, which doesn't happen very often.

The new throttling should not have a negative impact when the host is
online. Lookups against the network take some time and dials for all
results must complete or hit the cache before a new one is started. This
usually takes longer than four seconds, leaving online lookups
unaffected.

Fixes #1296

* BuyGas => SubGas
* RefundGas => AddGas
* SetGasPool => SetGasLimit

mist: R.I.P.

     /"""""/""""""".
    /     /         \             __
   /     /           \            ||
  /____ /             \           ||
 |     |  In Loving    |          ||
 |     |   Memory      |          ||
 |     |               |          ||
 |     |   2014-2015   |          ||
 |     |     * *   * * |         _||_
 |     |     *\/* *\/* |        | TT |
 |     |     *_\_  /   ...""""""| || |.""...."""""""".""
 |     |         \/.."""""..."""\ || /.""".......""""...
 |     |...."""""""........""""""^^^^"......."""""""".."
 |......"""""""""""""""........"""""...."""""..""-Jeff W.

ethtest improvements

eth/fetcher: lower max cache size, add timeout slack

unit test coverage for NewDataArgs

Disassociates hardcoded tests to skip when running via CLI. Tests still
skipped when running `go test`

eth: fix the propagation/announce order for mined blocks