Implement shutdown within sds

[Ivansete-status]

This substitutes this: #43

This is a temporary solution until we find a safer solution within nim-chronos, in status-im/nim-chronos#614.

Changes

• Undo temporary change of var ctxPool: seq[ptr SdsContext] in library/libsds.nim.
• Refactor resource cleanup in library/sds_thread/sds_thread.nim.
• Add new library/sds_thread/shutdown.nim module with the logic that should live in `nim-chronos.

Issue

• The aim is to also solve the following issue but undoing the temporary, yet interesting, change that was applied to overcome it: SDS usage crashes unit tests status-im/status-go#7216
• Maintenance Y2026H1 logos-delivery#3686

[Ivansete-status]

We will not implement this one because is not possible to cleanly close chronos main FDs. We will use a ctx pool instead.