Locale is very slow to import strings

Problem/Motivation

When importing existing configuration during site install, importing translated strings is very slow

Drupal\local\Gettest::fileToDatabase generates a call to Drupal\locale\PoDatabaseWriter::importString for each string which is then individually written to the database - in my case, for 6 languages, this caused an additional 51,000+ database writes on install and the installer takes around 10m to complete.

Proposed resolution

Support adding more than 1 translated string at a time to the database

Remaining tasks

tbd

API changes

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Release notes snippet

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