Deprecate RPC getblocknumber

This RPC is exactly identical to getblockcount. This duplication
dates back to commit 22f721dbf23cf5ce9e3ded9bcfb65a3894cc0f8c when
Satoshi created the RPC interface.

There's no need to have both, so we standardize on "count" which
matches the naming convention in getconnectioncount.

Following the tradition established with previously deprecated APIs,
getblocknumber continues to work, but it's not listed in the help
system.