LastCacheProvider is the API used to create last caches and write
table batches to them. It uses a two-layer RwLock/HashMap: the first for
the database, and the second layer for the table within the database.

This allows for table-level locks when writing in buffered data, and only
gets a database-level lock when creating a cache (and in future, when
removing them as well).

Added basic APIs on the write buffer to access the last cache and then a
test to the last_cache module to see that it works with a simple example

Addressed three parts of PR feedback:

1. Remove double-lock on cache map
2. Re-order the get when writing to the cache to be outside the loop
3. Move the time check into the cache itself

This refactors the last cache to use a nested caching structure, where
the key columns for a given cache are used to create a hierarchy of
nested maps, terminating in the actual store for the values in the cache.

Access to the cache is done via a set of predicates which can optionally
specify the key column values at any level in the cache hierarchy to only
gather record batches from children of that node in the cache.

Some todos:
- Need to handle the TTL
- Need to move the TableProvider impl up to the LastCache type

This re-writes the datafusion TableProvider implementation on the correct
type, i.e., the LastCache, and adds conversion from the filter Expr's to
the Predicate type for the cache.

Last caches will have expired entries walked when writes come in.

Changed key columns so that they do not accept null values, i.e., rows
that are pushed that are missing key column values will be ignored.

When producing record batches for a cache, if not all key columns are
used in the predicate, then this change makes it so that the non-predicate
key columns are produced as columns in the outputted record batches.

A test with a few cases showing this was added.

Ensure key columns in the last cache that are not included in the
predicate are emitted in the RecordBatches as a column.

Cleaned up and added comments to the new test.

Added two tests, as per commit title. Also moved the eviction process
to a separate function so that it was not being done on every write to
the cache, which could be expensive, and this ensures that entries are
evicted regardless of whether writes are coming in or not.

CacheAlreadyExists errors were only being based on the database and
table names, and not including the cache names, which was not
correct.

This also adds explicit support for series key columns to distinguish
them from normal tags in terms of nullability

A test was added to check nulls work

Support the addition of new fields to the last cache, for caches that do
not have a specified set of value columns.

A test was added along with the changes.