fix lock usage

[ericphanson]

closes #31

There are already tests which should cover this, but they don't seem to be causing issues on main. I did check that the original example from #31,

import Memoize: @memoize
import LRUCache: LRU

@memoize LRU{Int,Int}(maxsize=1000) foo(i::Int) = i

foo(5)

does now work (it throws an error no matter how many times I call it), whereas for me on main it throws an error, but then a second invocation of foo hangs.

Here I use Base.@lock, which is document+exported on Julia 1.9+, so should be fine to use here (non-internal).
Nevermind, @lock isn't defined on 1.0, so I'll just use manual finally calls.

[ericphanson]

CI failure seems unrelated