* internal/auth/password: add resizable pool
The resizable pool wraps the permit pool with a mutex
to allow it to be resized at runtime.
* internal/auth/password: add concurrency limit
The default concurrency limit is 1, and it can be
set using the SetHashingPermits function.
* internal/cmd/config: allow configuring hashing limits
The new concurrent_password_hash_workers configuration
value can be used to loosen the constraints on the
userpass auth method hashing operation.
* internal/cmd: set password hashing constraints
Sets the password hashing concurrency constraints on
startup and config reload.
* CHANGELOG: add notice about password concurrency limit
Johan Brandhorst-Satzkorn