lxd/db: Add custom volume snapshot functions
lxd/db: Add custom volume snapshot functions

This introduces some new functions

	* StoragePoolVolumeSnapshotCreate
	* StoragePoolVolumeSnapshotUpdateByProject
	* StorageVolumeSnapshotExpiryGet

The first two are similar to their non-snapshot counterparts,
except that both take an additional expiry date argument.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

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