Add key_hash for smaller index

[djmb]

This adds a key_hash column to the solid_cache_entries table. This column is a 64-bit integer that is a truncated SHA256 hash of the key. This allows us to use a smaller index than the 1024 byte key column.

The byte_size column is also added to the table. This is the size of the value in bytes. This is for a separate feature but is included in this commit to piggy back on the migrations.

The migrations themselves are split into three steps. This allows existing tables to be migrated without downtime.

There is a new configuration field key_hash_stage that can be set to :ignored, :unindexed or :indexed. This controls how the key_hash column is used and should be set as the migrations are completed.

The default is :indexed, which is what is required for fresh installations of solid_cache. In this case you can apply all three migrations at the same time.

In the case of an existing setup however, the steps are:

1. Install the new gem, install the migrations and set config.solid_cache.key_hash_stage = :ignored
2. Run the first migration, which adds the new columns
3. Update config.solid_cache.key_hash_stage = :unindexed
4. Backfill or truncate the table
5. Run the second migration, which adds the indexes and null constraints
6. Update config.solid_cache.key_hash_stage = :indexed
7. Run the third migration, which removes the old key index

For later versions of the gem we'll assume that the key_hash column exists and the config.solid_cache.key_hash_stage. We'll also squash the migrations into a single file.