Add a common prefix for titan keys

[shafreeck]

Add a configurable option to indicates the prefix of all titan keys, this can be used titan together other services or deploy separate multiple titan services on the same TiKV cluster.

[ironsalsa]

+1 to this - currently, I have to run multiple full TiKV+Titan stacks for separate applications.

[shafreeck]

@ironsalsa Thanks for trying titan!