Newly elected raft leader should wait for in flight blocks
to be committed, before accepting new envelopes and creating
new blocks. Otherwise all those blocks created would be uncle
blocks and we don't permit this situation in Fabric.

Change-Id: Ia5adac185263735eace1fc805ebea0f5c98b2fb1
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>