SERVER-9658 Reduce frequency of debug message
SERVER-9638 Handle case of previous moveChunk not finished yet

Depending on the speeds of operations in the shards, it is possible that
the "to" shard has not finished a migration when we try the next one.  Use
assert.soon() to wait a little and retry when this happens.
SERVER-9502 prohibit _id as RegEx

Otherwise, this breaks when replication attempts to do a query with the regular expression as _id.
SERVER-9502 fix assert code
SERVER-9502 fix assert code in unit test