ReplDev: potential crash in HomeStore shutdown

[yuwmao]

In homestore shutdown process, repl dev is released before statemachine(also held by logstore), and repl dev doesn't aware of the statemachine status.
In baseline resync, read_snapshot is async and might access replDev, so reading snapshot and shutdown happen simultaneously, crash may occur.

[JacksonYao287]

@yuwmao I checked the pg_blob_iterator.cpp and find repl_dev is used in two places:

1 repl_dev_->get_blk_size(): this will not be tracked by graceful shutdown, so can you make this a member of PGBlobIterator? which means when the PGBlobIterator is created , it knows the blk_size, so that no need to query repl_dev for blk_size every time.

2 repl_dev_>async_read, this is already tracked by graceful shutdown, since it will call data_service.asycn_read