Basic fixes to calibre-sync (#3558)

* Properly create intermediate directories when receiving books from Calibre.

This fixes an issue where you can't receive books except into directories that
already exist on the Kobo, which, in particular, causes problems when your
configuration in Calibre is something like "put books in $Author/$Title.epub"
and you haven't previously synced any books by that author.

* Wake up periodically to process ZMQs if any are registered.

This fixes an issue where if there are any timed events (such as the suspend
timer) in the queue, ZMQ events may not get processed until the timed event
fires, which is a problem when (for example) the suspend timer goes off in
an hour and you have something trying to send a book to the kobo over wifi
*right now*.

With this change, the event loop will wake up every 50ms to check for ZMQ
events and process them if necessary. If there are no ZMQs registered (which
is typical), it uses the original behaviour -- so this won't affect battery
life under normal usage.

plugins/calibrecompanion.koplugin/main.lua

@@ -5,6 +5,7 @@ local JSON = require("json")
 local DEBUG = require("dbg")
 local _ = require("gettext")
 local NetworkMgr = require("ui/network/manager")
+local util = require("frontend/util")
 
 require("ffi/zeromq_h")
 
@@ -320,6 +321,7 @@ function CalibreCompanion:sendBook(arg)
     local inbox_dir = G_reader_settings:readSetting("inbox_dir")
     local filename = inbox_dir .. "/" .. arg.lpath
     DEBUG("write to file", filename)
+    util.makePath((util.splitFilePathName(filename)))
     local outfile = io.open(filename, "wb")
     local to_write_bytes = arg.length
     local calibre_device = self