blockstore: Call LedgerColumn methods instead of Database

[steviez]

Problem

#3603 is looking to introduce a change where the rocksdb keys (byte reference) will be stack-allocated instead of heap-allocated. In order to accomplish this, any method that provides a key (ie just about all of them) need to go through the LedgerColumn object since each LedgerColumn has a constant key length that is known at compile time.

On the other hand, the Database functions that operate on a Column take a Column as a generic parameter. Since that function is generic over any column, the key must be heap allocated as we can't know what the key length will be until invocation. So, we need to phase out the Database calls in order to work towards the stack allocated keys.

This PR pulls breaks several commits out of #3603 to keep reviews smaller

Summary of Changes

Use the LedgerColumn objects in Blockstore to perform get() and iter() calls instead of getting it from the Database object directly

[cpubot]

LGTM

[steviez]

This change is fairly innocuous, so going to push despite not getting several more ship-its; will definitely get more sets of eyes on the more interesting changes