adb: fix sync.

adbd's file sync service doesn't handle a full socket gracefully,
immediately terminating the service as soon as it fails to write a
response. This would generally be fine if the socket's buffer were as
large as it claims (212992 by default with a 64-bit kernel), but this
buffer size is a giant lie, as each write has 576 bytes of overhead
that's used up in the send buffer. When setting the send buffer size,
the kernel helpfully doubles the value to attempt to account for the
overhead, but when writing 8 byte responses, only 2% of the buffer
actually gets used for responses, so we run out of buffer after 364
files instead of the 26624 that would be expected.

Fix this by processing the responses as they become available, and
calculate a maximum limit to how many sends we dispatch before we stop
and wait for responses to come in.

Test: manually modified adbd to respond with giant error messages, and
      modified adb to not read responses until we choose to block
Change-Id: Ieb8c935662864211e2fd16c337ffed0992990086