snapuserd: Fix merging of overlapping copy blocks

As part of r.android.com/c/1678745/7, overlapping
copy operations was allowed to batch merge which
is not right. The intention of that CL was
to avoid un-necessary write traffic involved
in flushing data to scratch space. However,
as part of the optimization, copy operations
were merged. More specifically, the problem
comes into play when the number of overlapping
copy operations is more than the read-ahead
window size (2MB).

I have added a new test case to test this
specific code path to avoid future regressions.

Additionally, remove un-necessary "send()"
as part of "detach" response from snapuserd server.
Client is not waiting for any response. It
just creates a race window which is harmless
but error log will be misleading.

Bug: 187506548
Test: cow_snapuserd which tests the similar
configuration as seen in the COW file in the bug
report.
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ic0f1ddd390f79966aabfbeadb7d64bc5bb86e83b