Reduce P2Pool latencies with several payout addresses and large block templates

[gyver]

It seems that P2Pool takes some time generating work with high number of transactions which is made worse on public nodes where the work must be generated for several payout addresses. This can lower efficiency significantly and should either be fixed or documented as a limitation.

On private nodes like mine (only allowing miners sharing the same payout address) this isn't really noticeable (probably <100ms even with maxed out blocks to serve all miners) but on public nodes like zvs' (on bitcointalk) one the delays between each work generation add up and the last one can be seriously lagging (by several hundreds of ms and probably in some extreme cases several seconds).

Here's the post of zvs on bitcointalk with logs:

https://bitcointalk.org/index.php?topic=153232.msg2499751#msg2499751