Add extended 'try' mode for completion-style I/O

This patch allows completion-style I/O to have 'immediate write
completion'. This is often when there is enough buffer space in the
socket to write data immediately without needing to allocate a new
buffer and write structure to perform a so-called asynchronous write.
Currently this feature is not implemented in any of the plugins and the
interface may change in the future.

Change-Id: I9407b46dfdc88d1baf01d1ed3b734f0497cb888d
make server->curhost a pointer

This wastes 1k of memory within the server structure and it's barely
ever accessed.

Change-Id: I6db4676138e810e8890f3a1b40b0be800811e26c
lcbio: optimize field layout and access patterns

- Move some fields closer together in lbcio_CTX
- Use cached fd field in CTX rather than ->sock. This makes it so that
  we never actually have to reference the socket structure in normal
  usage

Change-Id: Idd8cfbf7d4ec0cedb8f70dee1745bbb8bc263218
travis test..

Change-Id: I0b2c34b6c316b2dc5be3139d6014ba3618599574
lcb3 response API stubs

This commit adds some stubs for the response API. none of these APIs are
implemented yet.

Change-Id: Ie1e2d9919b0ae4e0ed6974f67fc3f9d8e8a65c00