adb: add nonblocking fd Connection.

Implement a nonblocking version of FdConnection. The initial
implementation will be somewhat slower than the blocking one for large
packet sizes, due to an extra copy when coalescing an IOVector into an
apacket, but is still substantially faster for small packets.

Test: adb_benchmark
Change-Id: I4900c9ddf685d3bd557b8cb43958452ecb23db53