fix: move from semaphore to rwmutex

[zach-source]

Description of your changes

Replaced the semaphore in the internal terraform harness with a RWMutex. RWMutex protects the terraform shared cache from corruption. If an init is performed, it requires a write lock. Only one write lock at a time. If another action is performed, a read lock is acquired. More than one read locks can be acquired. This prevents an init from inadvertently changing a plugin while it's in use. Prevents issues with text file busy errors. This is required if using max-reconcile-rate greater than 1.

Fixes #186

I have:

• [x] Run make reviewable to ensure this PR is ready for review.

How has this code been tested

I've tested the code locally with a max reconcile rate of 4.

[Upbound-CLA]

All committers have signed the CLA.

[ytsarev]

/test-examples="examples/workspace-inline-aws.yaml"