Spec additional Client API features in V2

[jungkees]

We decided to not expose reserved client that closed #1228 #1215 #1123 #1078 #1035 #1034 #1031 #1030.

For V1,

Remove:

• Client.reserved.
• ClientQueryOptions.includeReserved.
• FetchEvent.reservedClientId.
• FetchEvent.targetClientId.
• FetchEventInit.reservedClientId.
• FetchEventInit.targetClientId.

Do:

• Clean up the steps of Clients interface methods.

For V2,

Spec:

• FetchEvent.resultingClientId, FetchEvent.replacesClientId, and the underlying behavior including the execusion ready flag as discussed in consider Client behavior for windows where initial about:blank is replaced with a loaded document #1091 (comment).
• Clean up the steps of Clients interface methods including making clients.get() resolves when the client is execution ready; making sure clients.claim() algorithm to not claim reserved clients.

EDIT: Change the name of FetchEvent.targetClientId (for V2) to FetchEvent.replacesClientId as per #1091 (comment).