community: Chroma - remove reference to collection upon delete_collection

[tazarov]

Description:

• Reference to Collection object is set to None when deleting a collection delete_collection()
• Added utility method reset_collection() to allow recreating the collection
• Moved collection creation out of __init__ into __ensure_collection() to be reused by object init and reset_collection()
• _collection is now a property to avoid breaking changes

Issues:

• chroma-core/chroma#2213

Twitter: @t_azarov