Deprecate estimatefee in v0.16, for final removal in v0.17.
This commit introduces a phased removal of RPC methods. RPC method is
disabled by default in version x, but can be enabled by using the
-deprecatedrpc=<method> argument. RPC method is removed entirely in
version (x+1).
[rpc] [tests] Add deprecated RPC test
This is a backport of Core PR11031