Add leading zeros to a block height while saving to S3

[khorolets]

AWS S3 doesn't have folders. All the objects stored in S3 bucket have keys and we can prepend the names of the objects with a slash to simulate folders.

When we list objects from S3 they are returned in alphabetically ascending order. The important part is that keys are strings and for example "19000000" is going before "9000000" so we need to make those keys with the leading zeroes. For instance, with 5

Thus "19000000" transforms to "000019000000"
And "9000000" to "000009000000"