BFT Block Puller: seek content type "header+sig" to send full config blocks

[tock-ibm]

• When seek-info content type is "header+sig", send full config blocks, but data blocks with block.Data=nil.
• Optimize IsConfigBlock() to return false fast when it is a block with more than 1 TX (the common case in high throughput networks). If there is 1 TX, go all the way to the channel header as usual.

[tock-ibm]

To send config blocks, change the code at
common/deliver/deliver.go L331

Use protoutil.IsConfigBlock(block)

Optimize protoutil.IsConfigBlock(block) to return false early by looking if the number of TXs at the block.

[tock-ibm]

done