blockstore: Make WriteBatch operations go through LedgerColumn

[steviez]

Problem

• rocksdb keys are currently allocated as Vec's; these allocations occur very frequently
• blockstore: Move rocksdb keys from heap to stack #3603 looks to make rocksdb key allocation happen on the stack to avoid the frequent heap allocation
• rocksdb keys can be stack allocated if we perform that logic within LedgerColumn since each column has a fixed-and-known-at-compile-time key length
• So, we need to shift logic that creates the rocksdb byte array keys into LedgerColumn

Summary of Changes

Shift the logic of converting keys from the Index type to bytes from WriteBatch to LedgerColumn, and add methods to LedgerColumn to add puts and deletes into a WriteBatch

This is similar to #3650, with the goal of making #3603 (which will contain the more interesting changes) a smaller diff

[bw-solana]

LGTM. Left 1 nit.

Curious if you were able to observe any speedup in profiling or anything after these changes

[steviez]

Curious if you were able to observe any speedup in profiling or anything after these changes

I wouldn't expect any speed up from this one; this PR is still mostly just shuffling things around as prep for #3603. 3603 is the one that actually swaps from heap to stack.