P2P: Limit requested blocks to avoid max-reversible-blocks

[heifner]

Currently, sync-fetch-span provides an upper limit on how many blocks ahead of applied blocks (chain_head) are allowed to be requested. As it is today, the maximum allowed number of unapplied blocks that can be requested/received is 2 * sync-fetch-span - 1. A large sync-fetch-span or a small max-reversible-blocks option can cause a node to shutdown if the accumulated reversible blocks exceeds the max-reversible-blocks.

Limit net_plugin requested blocks to the minimum of max-reversible-blocks - forkdb.size() and sync-fetch-span to avoid hitting the max-reversible-blocks during P2P syncing.

max-reversible-blocks PR is #493