Do not timeout P2P connections when creating a snapshot

[heifner]

Creating a snapshot ties up the main thread for the time it takes to create the snapshot. During that time, new blocks can't be processed by the main thread. This can cause the net_plugin's sync timer and/or block timer to fire and disconnect from its peers. The connection timer then has to fire for the connections to be re-established. This can cause a node that is syncing and periodically creating snapshots to pause after each snapshot waiting for connections to be re-established and syncing to continue.

[bhazzard]

Possible solution may entail: Block timers during syncing should only be active when there is a pending request for blocks.