Add thread-safe cache

[fishi0x01]

• replaces all thread-unsafe caching mechanisms with a thread safe cache
• add a vault test with many secrets, for better concurrency testing

Having a thread safe cache will make concurrency implementation easier.

Fixes #82

[fishi0x01]

Performance test extends CI test time too much. Will adjust it to run only once and not on each KV.

[fishi0x01]

Concurrency tests now only run on single vault and KV version to save time.

[mattlqx]

👍🏻 This looks like a great change I can refactor my pull around.

[mattlqx]

Can't ok be used for this?

[fishi0x01]

Good catch 👍
Adjusted it now

[fishi0x01]

Sorry, apparently this doesn't work:

var hit bool
if value, hit := cache.listQueries[path]; hit {
    result = value.Secret
    err = value.Err
}
if hit {
    // this branch is never reached
    return result, err
}

Just now verified that it results in hit always being false after leaving the first if scope. I am not really sure why though

[mattlqx]

Maybe the assignment is considered in the scope of the if because of the :=? These little Golang things catch me out too. :)

[fishi0x01]

Now this works :)

value, hit := cache.listQueries[path]
if hit {
    return value.Secret, value.Err
}

I think this part ; ok { binds the two initialized variables solely to the if scope.

[fishi0x01]

@mattlqx Concerning concurrency:
I have another branch based on this PR, which parallelizes the Traverse and runCommandWithTraverseTwoPaths functions. It seems like mv, rm and cp can benefit quite well from it. replace and grep do not benefit from it though, because they use a different traversal logic - however, that can be adjusted.
Will open that PR soon

[fishi0x01]

#86