Patch FIXME: Fix concurrency in local cache

Patch FIXME: Fix concurrency in local cache

Tried to protect access via mutex, but oh boy. That escalated quickly.

Instead, restore tree workspace isolation. Since the 1-thread-per-TAL
architecture has survived, this allows the validation to merrily read
and write the local cache without any locking.

Each thread now builds its own resource table. The main thread joins
them.

This basically zeroizes resource sharing between validation threads.
Great from an engineering perspective, maybe not so much from the
performance angle.