Share one local and one remote DataFolder instance throughout the server

[CGodiksen]

This PR closes #416 by sharing a single local data folder and a single remote data folder throughout the server by wrapping them in an Arc. While looking into if there were any problems with sharing a single DataFolder instance, a small locking inefficiency was found when opening a DeltaTable. This is also fixed in this PR. Finally, the PR also fixes some naming inconsistencies, specifically regarding arguments being named local_data_folder while actually being a path or an object store.

[Copilot]

Pull request overview

This PR addresses issue #416 by switching server components to share a single local DataFolder and (when configured) a single remote DataFolder instance via Arc, reducing redundant construction of the DataFolder’s cache and DataFusion session context. It also improves Delta table cache access to avoid holding a DashMap shard lock across async I/O, and cleans up a few naming inconsistencies (e.g., “folder” vs “path/object store”).

Changes:

• Share DataFolder instances across the server using Arc<DataFolder> instead of cloning DataFolder.
• Avoid holding a DashMap guard while awaiting DeltaTable::load() when returning cached tables.
• Rename parameters/fields in a few places to better reflect what is being passed (paths/object stores).

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/modelardb_storage/src/data_folder/mod.rs	Removes Clone from DataFolder, stores a single SessionContext, and updates Delta table caching to avoid async I/O while holding a DashMap guard.
crates/modelardb_server/tests/integration_test.rs	Renames variables/params to reflect that a temp dir path is passed to the server CLI.
crates/modelardb_server/src/storage/uncompressed_data_manager.rs	Switches local_data_folder ownership to Arc<DataFolder> for shared use.
crates/modelardb_server/src/storage/uncompressed_data_buffer.rs	Renames “local_data_folder” object store parameters/fields to local_object_store for clarity.
crates/modelardb_server/src/storage/data_transfer.rs	Uses shared Arc<DataFolder> for local/remote folders; updates tests accordingly.
crates/modelardb_server/src/storage/data_storage_compactor.rs	Uses shared Arc<DataFolder> for the compactor’s local folder; updates tests accordingly.
crates/modelardb_server/src/storage/compressed_data_manager.rs	Stores the manager’s local_data_folder as Arc<DataFolder> and updates constructor/tests.
crates/modelardb_server/src/data_folders.rs	Makes DataFolders hold Arc<DataFolder> and updates try_from_args() to construct shared instances.
crates/modelardb_server/src/context.rs	Updates tests to wrap DataFolder in Arc before constructing DataFolders.
crates/modelardb_server/src/configuration.rs	Changes ConfigurationManager to hold Arc<DataFolder>; updates tests that construct DataFolders.
crates/modelardb_server/src/cluster.rs	Updates Cluster to store remote_data_folder as Arc<DataFolder> and adjusts tests/helpers to avoid cloning DataFolder.

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