Reset peerip only if remote_addr_header is set

[calvinxiao]

Description

In tcp mode when both remote_addr_value and remote_addr_header are not set, client.peerip method
calls @io.peeraddr in every request, I dont't think we need to reset @peerip to nil in such condition.

@io.peeraddr uses two syscalls getpeername and recevfrom, by caching the @peerip in Client instance,
we can save two syscalls on every request in an established connection, except for the first request.

hello.ru performance improvement:

• master, 4107 RPS
• caching peerip, 5224 RPS

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