BlockSender performance regression due to volume scanner edge case

HDFS-11160 attempted to fix a volume scan race for a file appended mid-scan by reading the last checksum of finalized blocks within the BlockSender ctor. Unfortunately it's holding the exclusive dataset lock to open and read the metafile multiple times Block sender instantiation becomes serialized.

Performance completely collapses under heavy disk i/o utilization or high xceiver activity. Ex. lost node replication, balancing, or decommissioning. The xceiver threads congest creating block senders and impair the heartbeat processing that is contending for the same lock. Combined with other lock contention issues, pipelines break and nodes sporadically go dead.