Remove EtcdLock and apply breaking changes to lock interface

[JulianVennen]

Changes

• Lock now implements LockInterface
• setBreakOnDestruct, setIdentifier and update now return the lock instance ($this)
• break() no longer performs unnecessary requests if the lock is not present
• Added getRemainingLockDuration which returns the remaining duration a lock is valid for.

Breaking Changes

• Removed EtcdLock. Use Lock and Lock->lock() instead.
• The Constructor of LockInterface no longer acquires the lock. Use ->lock() instead
• The parameters for lock, refresh and waitForOtherLocks are now properties of the lock itself.
• Lock->isLocked() now returns true if the lock is still valid. To get the remaining duration of the lock, use getRemainingLockDuration()
• Lock->break() and Lock->removeLock() now return void instead of always returning true
• Lock->$identifier is no longer nullable, but a null value can still be passed in the constructor to use the default identifier
• Lock->getClient() is now static