Main: Use lock files in data dir + CL dir to ensure single instance.
Main: Use lock files in data dir + CL dir to ensure single instance.

Acquires and maintains lock files during execution. 
Lock files are deleted on "clean" exit, and re-taken on uncontended 
startup.

Fixes #34

scylladb/scylla,glommer/scylla,asias/scylla,rentongzhang/scylla,victorbriz/scylla,wildinto/scylla,senseb/scylla,dwdm/scylla,victorbriz/scylla,scylladb/scylla,wildinto/scylla,justintung/scylla,duarten/scylla,avikivity/scylla,guiquanz/scylla,duarten/scylla,capturePointer/scylla,rluta/scylla,rluta/scylla,tempbottle/scylla,avikivity/scylla,respu/scylla,bowlofstew/scylla,guiquanz/scylla,glommer/scylla,aruanruan/scylla,scylladb/scylla,senseb/scylla,eklitzke/scylla,wildinto/scylla,kjniemi/scylla,tempbottle/scylla,aruanruan/scylla,capturePointer/scylla,shaunstanislaus/scylla,stamhe/scylla,respu/scylla,kangkot/scylla,justintung/scylla,dwdm/scylla,raphaelsc/scylla,bowlofstew/scylla,scylladb/scylla,kangkot/scylla,acbellini/scylla,kangkot/scylla,linearregression/scylla,linearregression/scylla,guiquanz/scylla,rentongzhang/scylla,respu/scylla,eklitzke/scylla,stamhe/scylla,shaunstanislaus/scylla,tempbottle/scylla,phonkee/scylla,phonkee/scylla,kjniemi/scylla,raphaelsc/scylla,shaunstanislaus/scylla,rentongzhang/scylla,victorbriz/scylla,acbellini/scylla,gwicke/scylla,kjniemi/scylla,dwdm/scylla,glommer/scylla,stamhe/scylla,avikivity/scylla,acbellini/scylla,gwicke/scylla,capturePointer/scylla,justintung/scylla,linearregression/scylla,senseb/scylla,bowlofstew/scylla,aruanruan/scylla,gwicke/scylla,raphaelsc/scylla,phonkee/scylla,duarten/scylla,asias/scylla,rluta/scylla,asias/scylla,eklitzke/scylla