Retain uuid and _core keys when replacing configuration

Problem/Motivation

When default configuration is installed, changes are made that minimally include (a) adding a uuid value and (b) setting a hash of the default config to a _core key.

A full replacement of the target configuration item can be triggered by adding a special key-value pair, config_rewrite = 'replace', to an item added to config/replace. However, there is no code as yet to retain the keys added on original configuration install.

Proposed resolution

In _config_rewrite_rewrite_config(), within the if clause, copy over the relevant values:

foreach (['_core', 'uuid'] as $key) {
  if (isset($original_config[$key])) {
    $rewrite[$key] = $original_config[$key];
  }
}

Alternately, introduce a dependency on Configuration Update Base (config_update) from the Configuration Update Manager project and use the ConfigReverter::revert() method. See #2829506: Simple config doesn't retain config hash when reverted. However, the ::revert() method wouldn't be directly applicable, as we can't feed in the replacement data.

Remaining tasks

User interface changes

API changes

Data model changes