Static field info caches cannot be rebuilt during the same requests

Problem/Motivation

field_encrypt.inc statically caches field info (metadata) using PHP's native static variables. While this probably serves to achieve the desired performance improvement, such caches can never be modified from outside the functions they are defined in. In practice, it means they are not kept up-to-date with system changes during the same request, and their functions will act on stale data, causing bugs that are hard to find and fix.

Proposed resolution

Use drupal_static(), which is Drupal 7's solution to flushable static caches in procedural contexts. Such caches can easily be cleared by calling drupal_static_reset(), which is what core does to keep all static caches up-to-date with the system during the same request.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.