txbench fixups

Change-Id: I2a1c5213a9c185bc9b5807628e10c63c5480812e
Don't unlock GIL if destroyed (callbascks.c)

Change-Id: I79294d185483f0fdd27aa6fae63029b05d096b28
Make destruction handling a bit clearer to user

Change-Id: I08e6e55722b6c291a34c7b42f68240551417010a
Don't recreate mock each time

Change-Id: I8af38bb2deb22fd22e387b4253f3e72ecb2217df