To kick off our (tsdb) storage backends this adds our first implementing
a new `Storage(Protocol)` client interface. Going foward, the top level
`.storage` pkg-module will now expose backend agnostic APIs and helpers
whilst specific backend implementations will adhere to that middle-ware
layer.
Deats:
- add `.storage.marketstore.Storage` as the first client implementation,
moving all needed (import) dependencies out from
`.service.marketstore` as well as `.ohlc_key_map` and `get_client()`.
- move root `conf.toml` loading from `.data.history` into
`.storage.__init__.open_storage_client()` which now takes in a `name:
str` and does all the work of loading the correct backend module, its
config, and determining if a service-instance can be contacted and
a client loaded; in the case where this fails we raise a new
`StorageConnectionError`.
- add a new `.storage.get_storagemod()` just like we have for brokers.
- make `open_storage_client()` also return the backend module such that
the history-data layer can make backend specific calls as needed (eg.
ohlc_key_map).
- fall back to a basic non-tsdb backfill when `open_storage_client()`
raises the new connection error.
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