Rename BatchBoundaries::start to be "begin"

The member inside BatchBoundaries is called "start", but the corresponding field inside the minvalid doc is called "begin". This mismatch hinders the readability of the code, especially for people who are unfamiliar with it.

It also means that some error messages refer to the "start" timestamp, even though this timestamp is called "begin" in minvalid, eg. this one:

if (!mv.start.isNull()) {

    // If we are in the middle of a batch, and recoveringm then we need to truncate the oplog.

    LOG(2) << "Recovering from a failed apply batch, start:" << mv.start.toBSON();

    truncateOplogTo(txn, mv.start.getTimestamp());

}

Since changing the minvalid field name would be needlessly difficult, it would be good to change the relevant occurrences of "start" in the code to "begin".