Code cleanups

[rkuris]

• Added _ for large numbers for readability
• Added some comments, removed obvious ones
• Move prometheus stats (into stats struct)
• Added error handling for os.Getwd() et al
• Switch to fmt.Errorf and printf at caller
• use 'range INT` instead of usual for loop

Why this should be merged

Cleaner code for blog post

How this works

• Large numbers support _ as placeholders to improve readability link
• Having variables grouped in a struct idenfies them as soley metrics for readability (metrics.batches vs batchCounter)
• Although os.Getwd is unlikely to fail, it isn't impossible, and it's best practice to at least panic if it fails
• fmt.Errorf returns an error, which can then be returned to the caller link
• range over integer is easier to read and functionally equivalent link

How this was tested

Ran bench locally, still seems to work:

➜  benchmarks_eth git:(rkuris/bench_merkledb) go run . --n 1000 --v
Initializing database. done!
Generated and inserted 0 batches of size 10000 in 4.153792ms
Completed initialization with hash of 0x788272ac157b96c5af259c14657cfa0f5b261c4d07fac7cbdfba53148ad0960e
delete rate [167]       update rate [289]       insert rate [585]       batch rate [47]
delete rate [117]       update rate [259]       insert rate [417]       batch rate [36]