Update dcrwallet mod

[matheusd]

~Currently rebased on top of #85~

This updates dcrlnd to use the latest decred.org/dcrwallet module.

Updated

It now also:

• Brings back the (faster) simnet miner which requires the use of the regentemplate json-rpc command in dcrd
• Removes some unnecessary (in the context of dcrlnd) wallet loading options
• Reduces the timeout for the sweeper component in itests (which makes most tests faster)
• Fixes all sorts of itest/unit test failures triggered by the faster blocks/wallet/sweeper

It seems there are still some flakiness for tests involving chain notifications but I'll reassess them during/after #83.

[matheusd]

Addressed review items, plus updated to latest dcrwallet commit with blockcf2 support.