Lock dimension slices when creating new chunk

[erimatnor]

This is a backport of an earlier PR, now targeting 1.7.x.

This change makes two changes to address issues with processes doing
concurrent inserts and drop_chunks calls:

• When a new chunk is created, any dimension slices that existed prior
  to creating the new chunk are locked to prevent them from being
  dropped before the chunk-creating process commits.

• When a chunk is being dropped, concurrent inserts into the chunk
  that is being dropped will try to lock the dimension slices of the
  chunk. In case the locking fails (due to the slices being
  concurrently deleted), the insert process will treat the chunk as
  not existing and will instead recreate it. Previously, the chunk
  slices (and thus chunk) would be found, but the insert would fail
  when committing since the chunk was concurrently deleted.

A prior commit (PR #2150) partially solved a related problem, but
didn't lock all the slices of a chunk. That commit also threw an error
when a lock on a slice could not be taken due to the slice being
deleted by another transaction. This is now changed to treat that case
as a missing slice instead, causing it to be recreated.

Fixes #1986