Version 1.6.0 Feature Release

After a very long time indeed (over 2 years!), we are happy to announce v1.6.0 of NNG!

• Numerous critical bugs were fixed; some of these led to seg faults, crashes, and
  memory leaks. See bugs #1523, #1713, #1702, #1657, #1347, #1518, #1526, #1541, #1638, #1543, #1657, #1658

• Significant performance optimizations have been made, especially to the BUS protocol, the
  nng_sendmsg() and nng_recvmsg() functions, when connecting and disconnecting lots of pipes,
  and when using very different expiration times with vast numbers of requests.

• New APIs were added for nng_aio_busy(), nng_ctx_sendmsg(), nng_ctx_recvmsg(), nng_device_aio().

• A CMake tunable for limiting the number of threads use for request expiration is provided
  via the NNG_MAX_EXPIRE_THREADS option.

• Additionally various fixes for compilation problems, documentation errata, test case, and so forth
  have been applied.