Add a read_full_buffer argument to ConnectionFileDescriptor::Read

Summary:
AdbClient was attempting to handle the case where the socket input arrived in pieces, but it was
failing to handle the case where the connection was closed before that happened. In this case, it
would just spin in an infinite loop calling Connection::Read. (This was also the cause of the
spurious timeouts on the darwin->android buildbot. The exact cause of the premature EOF remains
to be investigated, but is likely a server bug.)

Since this wait-for-a-certain-number-of-bytes seems like a useful functionality to have, I am
moving it (with the infinite loop fixed) to the Connection class, and adding an
appropriate test for it.

Reviewers: clayborg, zturner, ovyalov

Subscribers: tberghammer, danalbert, lldb-commits

Differential Revision: http://reviews.llvm.org/D19533

llvm-svn: 268380