lua: Use a global lock to call Lua's API

lua: Use a global lock to call Lua's API

Since vanilla Lua isn't multi-thread-safe, a global lock should be used
when calling their functions. The previous code causes unstable behavior
especially on loading Lua's C modules with both read & write threads.

e.g.)

  collectd.register_read(function()
    require('cqueues')
    return 0
  end)

  collectd.register_write(function()
    return 0
  end)

Note that you need to add the following config to your collectd.conf to
load C modules:

  <LoadPlugin lua>
    Globals true
  </LoadPlugin>

Signed-off-by: Takuro Ashie <ashie@clear-code.com>
Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>