[332] Rename snapshot handlers + add optimistic locking

[drieshooghe]

• :recycle: rename snapshot-handlers to snapshot-repositories
• ✨ add an event-store-version-conflict exception
• ✅ add tests for event version conflicts
• ✨ add optimistic version lock checks
• ✨ add optimistic locking on the in-memory event-store append-events method
• ✅ make the version-conflict tests more specific
• ✨ add a snapshot-store-version-conflict exception
• ✅ add tests for snapshot version conflicts
• ✅ refactor test variables
• ✅ refactor variables in tests for snapshot version conflicts
• ✨ add optimistic locking on the in-memory snapshot-store append-snapshot method

Description

• renames snapshot-handler to snapshot-repository
• adds optimistic locking to the event- and snapshot-stores

Fixes #332

Type of change

• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update