Bitshares txbuffer must be reset when switching workers

[bitphage]

1. Setup one SO worker with no key

2019-03-03 15:29:39,841 (worker:124) - worker6 - ERROR - in ontick()
Traceback (most recent call last):
  File "/home/vvk/devel/DEXBot-prod/dexbot/worker.py", line 122, in on_block
    self.workers[worker_name].ontick(data)
  File "/home/vvk/.local/share/virtualenvs/DEXBot-prod-N9mtHQyI/lib/python3.6/site-packages/Events-0.3-py3.6.egg/events/events.py", line 95, in __call__
    f(*a, **kw)
  File "/home/vvk/devel/DEXBot-prod/dexbot/strategies/staggered_orders.py", line 1952, in tick
    self.maintain_strategy()
  File "/home/vvk/devel/DEXBot-prod/dexbot/strategies/staggered_orders.py", line 296, in maintain_strategy
    self.execute()
  File "/home/vvk/devel/DEXBot-prod/dexbot/strategies/base.py", line 993, in execute
    r = self.bitshares.txbuffer.broadcast()
  File "/home/vvk/.local/share/virtualenvs/DEXBot-prod-N9mtHQyI/lib/python3.6/site-packages/bitshares-0.2.1-py3.6.egg/bitshares/transactionbuilder.py", line 402, in broadcast
    self.sign()
  File "/home/vvk/.local/share/virtualenvs/DEXBot-prod-N9mtHQyI/lib/python3.6/site-packages/bitshares-0.2.1-py3.6.egg/bitshares/transactionbuilder.py", line 380, in sign
    raise MissingKeyError
bitshares.exceptions.MissingKeyError

2. Error will be raised in another workers too because self.bitshares.txbuffer is common for all workers.

Soluition: reset txbuffer on error