Fix race condition in handing out value handle IDs for the LMDB backing store

[jorisdral]

Description

A race condition led to IDs for LMDB value handles being handed out more than once. Computing/handing out IDs is now atomic, so the race condition should not occur anymore.

Checklist

• Branch
  • Commit sequence broadly makes sense
  • Commits have useful messages
  • The documentation has been properly updated
  • New tests are added if needed and existing tests are updated
  • If this branch changes Consensus and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If this branch changes Network and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If serialization changes, user-facing consequences (e.g. replay from genesis) are confirmed to be intentional.
• Pull Request
  • Self-reviewed the diff
  • Useful pull request description at least containing the following information:
    • What does this PR change?
    • Why these changes were needed?
    • How does this affect downstream repositories and/or end-users?
    • Which ticket does this PR close (if any)? If it does, is it linked?
  • Reviewer requested

[jasagredo]

Looks good, thanks!!

[dnadales]

Thanks!