Fixed get_block_template_backlog performance

[SChernykh]

Before the fix, it processed all transactions in the mempool which could be very slow when mempool grows to several MBs in size. I observed get_block_template_backlog taking up to 15 seconds of CPU time under high mempool load.

After the fix, only transactions that can potentially be mined in the next block will be processed (a bit more than the current block median weight).