[LMP] Use datetime-initialized mt19937 instead of random_device.

Also, move it to be a class member. For #1707.
[LMP] Modify pos after querying original feature for #1707.
[LMP] Calculate features over immutable queue.

Otherwise position-dependent features may be the same for each next feature,
effectively removing the whole remainder of the list.

Finally fixes #1707.
Merge branch 'master' of github.com:0xd34df00d/leechcraft