Dont unlock more than one secret at a time

[DrPeterVanNostrand]

munlock() will unlock full pages of memory. Right now, we call munlock() upon every secret deallocation. This means that we may be preemptively unlocking secrets from RAM, if two secrets occupy the same page of memory, and one is dropped before the other.

Implement a counter that keeps track of the number of secrets allocated in each page of memory. Wait until a page's count is zero before calling munlock() on the page.

The counter should be incremented via ContainsSecret::mlock_secret(&self) and ContainsSecret::munlock_secret(&self). The counter must be thread safe.

[afck]

If we do #42, this issue is probably obsolete.

[afck]

Closing in favor of #42.