Deleting from a capped collection via collection scan reports number of documents deleted incorrectly

User deletes on capped collections became possible in SERVER-63201. However, when the delete on the capped collection is performed via a collection scan, a CappedPositionLost write error is returned back to the client. There are couple issues with this behavior:

• For limit:1 deletes (aka single-delete) 1 document is actually deleted but n:0 is returned in addition to the CappedPositionLost write error.
• For limit:0 deletes (aka multi-deletes) no documents are deleted, n:0 is returned in addition to the CappedPositionLost write error. The reporting here is accurate but we should probably make it possible to clear a capped collection.

[js_test:repro_capped_user_delete] 	"n" : 0,

[js_test:repro_capped_user_delete] 	"writeErrors" : [

[js_test:repro_capped_user_delete] 		{

[js_test:repro_capped_user_delete] 			"index" : 0,

[js_test:repro_capped_user_delete] 			"code" : 136,

[js_test:repro_capped_user_delete] 			"errmsg" : "CollectionScan died due to position in capped collection being deleted. Last seen record id: RecordId(1)"

[js_test:repro_capped_user_delete] 		}

[js_test:repro_capped_user_delete] 	],

[js_test:repro_capped_user_delete] 	"ok" : 1,