Replace rocksdb with relational database (eg sql) for queue and state db

Having a relational database with indexes might be better for #141. The new structure I'm envisioning would keep the statedb and use that as the queue by way of indexes:

Mode	state	progress	score	elapsed	remaining	hist	prev	next	in-memory-queue	won
lookup	= x
insert	...	...	...	...	...	...	...	...	...	...
update	= x		set min	set	set	set	set
process	= x							set	set false
evict	= x								set true
retrieve		= x	sort	sort	sort				= false, set true
recover									set false if true
solutions				sort						= true

• Our statedb lookups are for data at the primary key (state).
• As we process states, we insert the new states (or update the ones we've found faster routes to), and update the processed states with the next steps.
• Evicting from the in-memory-queue simply flips the bit.
• Retrieving states looks for states with the in-memory-queue bit set to false (and then sets them to true)
• On recovery, we can either retrieve all the elements marked as being in the memory queue, or set them all back to false and perform a retrieve.
• And we can retrieve solutions directly on recovery as well.