add Calibre Companion plugin

This plugin implements a simple Calibre Companion protocol that
communicates with Calibre Wireless Server from which users can
send documents to Koreader devices directly with WIFI connection.

Note that Calibre Companion(CC) is a trade mark held by MultiPie Ltd.
The Android app Calibre Companion provided by MultiPie is closed-source.
This plugin only implements a subset function of CC according to the
open-source smart device driver from Calibre source tree.

More details can be found at calibre/devices/smart_device_app/driver.py.

frontend/ui/message/streammessagequeue.lua

@@ -0,0 +1,83 @@
+local ffi = require("ffi")
+local DEBUG = require("dbg")
+local util = require("ffi/util")
+local Event = require("ui/event")
+local MessageQueue = require("ui/message/messagequeue")
+
+local dummy = require("ffi/zeromq_h")
+local zmq = ffi.load("libs/libzmq.so.4")
+local czmq = ffi.load("libs/libczmq.so.1")
+
+local StreamMessageQueue = MessageQueue:new{
+    host = nil,
+    port = nil,
+}
+
+function StreamMessageQueue:start()
+    self.context = czmq.zctx_new();
+    self.socket = czmq.zsocket_new(self.context, ffi.C.ZMQ_STREAM)
+    self.poller = czmq.zpoller_new(self.socket, nil)
+    local endpoint = string.format("tcp://%s:%d", self.host, self.port)
+    DEBUG("connect to endpoint", endpoint)
+    local rc = czmq.zsocket_connect(self.socket, endpoint)
+    if rc ~= 0 then
+        error("cannot connect to " .. endpoint)
+    end
+    local id_size = ffi.new("size_t[1]", 256)
+    local buffer = ffi.new("uint8_t[?]", id_size[0])
+    local rc = zmq.zmq_getsockopt(self.socket, ffi.C.ZMQ_IDENTITY, buffer, id_size)
+    self.id = ffi.string(buffer, id_size[0])
+    DEBUG("id", #self.id, self.id)
+end
+
+function StreamMessageQueue:stop()
+    if self.poller ~= nil then
+        czmq.zpoller_destroy(ffi.new('zpoller_t *[1]', self.poller))
+    end
+    if self.socket ~= nil then
+        czmq.zsocket_destroy(self.context, self.socket)
+    end
+    if self.context ~= nil then
+        czmq.zctx_destroy(ffi.new('zctx_t *[1]', self.context))
+    end
+end
+
+function StreamMessageQueue:handleZframe(frame)
+    local size = czmq.zframe_size(frame)
+    local data = nil
+    if size > 0 then
+        local frame_data = czmq.zframe_data(frame)
+        if frame_data ~= nil then
+            data = ffi.string(frame_data, size)
+        end
+    end
+    czmq.zframe_destroy(ffi.new('zframe_t *[1]', frame))
+    return data
+end
+
+function StreamMessageQueue:waitEvent()
+    local data = ""
+    while czmq.zpoller_wait(self.poller, 0) ~= nil do
+        local id_frame = czmq.zframe_recv(self.socket)
+        if id_frame ~= nil then
+            local id = self:handleZframe(id_frame)
+        end
+        local frame = czmq.zframe_recv(self.socket)
+        if frame ~= nil then
+            data = data .. (self:handleZframe(frame) or "")
+        end
+    end
+    if self.receiveCallback and data ~= "" then
+        self.receiveCallback(data)
+    end
+end
+
+function StreamMessageQueue:send(data)
+    --DEBUG("send", data)
+    local msg = czmq.zmsg_new()
+    czmq.zmsg_addmem(msg, self.id, #self.id)
+    czmq.zmsg_addmem(msg, data, #data)
+    czmq.zmsg_send(ffi.new('zmsg_t *[1]', msg), self.socket)
+end
+
+return StreamMessageQueue