allow uninitialization of lib handle

[sitano]

func Close() error

to allow releasing of the default library handle

[optnfast]

Thankyou for the PR. This needs a bit more before we can merge it.

1. Following PR2, I think it should use (*ResourcePool)Close to close each slot's sessions (replacing CloseSessions).
2. It needs documentation comments on each public method. See https://blog.golang.org/godoc-documenting-go-code for a style guide.
3. If you use ResetLibHandle but not Finalize, I think the system will be left in an inconsistent state, with a pool of session handles that don't belong to a future value of libHandle. So we need some strategy for preventing this situation, for instance having ResetLibHandle call Finalize first.

[sitano]

@nfewx I've refactored code based on your comments and mentioned PR #3.

I've created a new type sessionPool which secures a resourcePool with a RWMutex. The whole sessions pool made independent of the library singleton libHandle. I've added few helpers.

As the PKCS11Session type is public and can be created by any context, and the sessions pool is indepented of any specific context, and the session wrapper it self is kind of independent of the pool, I've placed Ctx *pkcs.Ctx into the session type, so it can implement Close().

[sitano]

I have removed vitess dependency and move resource pool implementation to this code base.

[optnfast]

I appreciate the desire to minimize dependencies but I'd rather keep that code as an external dependency, partly for licensing reasons and partly to simplify tracking of any upstream changes.

[sitano]

@nfewx ok. what do you think of new PKCS11Session struct? does it fit your expectations on design? another way was to put ctx object into the sessions pool object, but I was not sure whether the pool would be used across multiple contexts in the future i.e.

[sitano]

from another point of view, there is an API which recv handles which could be created with different session context (not libHandle) and then having it at hand allows passing of the right session handle to the pkcs11 calls.

[optnfast]

I like the design as of 4b969bb. Anyone else want to chip in before I merge? (@solcates?)