Do not use global HTTP connection to local CUPS

Before, a HTTP connection to the local CUPS was opened and kept in a
global variable of http_t* during the whole session. Functions
connecting to the local CUPS just used this connection.

Later on multi-threading for time consuming actions like DNS-SD
resolving and creating CUPS queues got introduced.

Also the sub-threads for creating CUPS queues used the global HTTP
connection. This could have cause drace conditions messing up the
http_t data structure.

Most probably this has caused to the "used" field of the data
structure (number of bytes which are in use in the internal buffer,
must be >= 0) getting negative nad this making the httpGets() function
of libcups busy-looping, which caused cups-browsed taking 100% of one
or two CPU cores and requiring SIGKILL to get terminated.

Bug reports at Ubuntu:

  - cups-browsed running non-stop on two cores
    https://bugs.launchpad.net/bugs/2049315

  - cups-browsed high-cpu usage
    https://bugs.launchpad.net/bugs/2067918

  - cups-browsed delays shutdown for 90 s
    https://bugs.launchpad.net/bugs/2073504

Bug report on CUPS:

  - OpenPrinting/cups#879

This commit does away with the global connection. Each function which
had used the global HTTP connection to the local CUPS creates an HTTP
connection onits own now and closes it before returning.CUPS can
accept several HTTP connections in parallel, therefore it is no
problem if we have individual connections now and if they happen in
parallel due to the sub-threads.

Note that testing is needed to determine whether this change actually
fixes the bug. The probability is high, butdue to the fact that the
bug is not easily reproducable it will take some time.