Add --init-db flag to create db if it does not exist

[Leopere]

• add --init-db flag
• if flag is set and db does not exist, create the db using the schema at https://github.com/lbryio/reflector.go/blob/master/db/db.go#L417-L452

[chrono]

Also, giving my 2 cents here... Wouldnt something like leveldb have made much more sense so we wouldnt have to run, monitor and secure a separate server process (mysql)? I'd love to hack around leveldb together with the reflector, but that doesnt make much sense to me as lond as S3 is a requirement.

[grin]

The short answer is that neither s3 nor mysql are requirements. Different backends are supported and new ones (like leveldb) are easy to add.

There's a longer discussion here: #28 (comment)