Add CompositeStorage which bundles multiple storages

[panarch]

e.g.

CREATE TABLE Foo ( .. ) ENGINE = MEMORY;
CREATE TABLE Bar ( .. ) ENGINE = SLED;

..

SELECT * FROM Foo JOIN Bar;

[coveralls]

Pull Request Test Coverage Report for Build 4095590144

• 444 of 458 (96.94%) changed or added relevant lines in 18 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 98.558%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
storages/composite-storage/src/lib.rs	49	52	94.23%
storages/composite-storage/src/store.rs	28	32	87.5%
storages/composite-storage/src/store_mut.rs	38	45	84.44%

Totals
Change from base Build 4040662072:	-0.002%
Covered Lines:	38415
Relevant Lines:	38977

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💛 - Coveralls

[ever0de]

Looks good 👍