Use raw time string from DB to generate ActiveRecord#cache_version

Currently, the `updated_at` field is used to generate a `cache_version`. Some database adapters return this timestamp value as a string that must then be converted to a Time value. This process requires a lot of memory and even more CPU time. In the case where this value is only being used for a cache version, we can skip the Time conversion by using the string value directly.

- This PR preserves existing cache format by converting a UTC string from the database to `:usec` format.
- Some databases return an already converted Time object, in those instances, we can directly use `created_at`.
- The `updated_at_before_type_cast` can be a value that comes from either the database or the user. We only want to optimize the case where it is from the database.
- If the format of the cache version has been changed, we cannot apply this optimization, and it is skipped.
- If the format of the time in the database is not UTC, then we cannot use this optimization, and it is skipped.

Some databases (notably PostgreSQL) returns a variable length nanosecond value in the time string. If the value ends in a zero, then it is truncated For instance instead of `2018-10-12 05:00:00.000000` the value `2018-10-12 05:00:00` is returned. We detect this case and pad the remaining zeros to ensure consistent cache version generation.

Before: Total allocated: 743842 bytes (6626 objects)
After: Total allocated: 702955 bytes (6063 objects)

(743842 - 702955) / 743842.0 # => 5.4% ⚡️⚡️⚡️⚡️⚡️

Using the CodeTriage application and derailed benchmarks this PR shows between 9-11% (statistically significant) performance improvement versus the commit before it.

Special thanks to @lsylvester for helping to figure out a way to preserve the usec format and for helping with many implementation details.

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