MB-11772: Add `release_free_memory` ioctl and `mcctl` program to access them

Add two new binary protocol commands:

  * PROTOCOL_BINARY_CMD_IOCTL_GET - Retrieve a property from the server
  * PROTOCOL_BINARY_CMD_IOCTL_SET - Modify a property on the server.

Using these, implement a `set release_free_memory` command which will
request the memory allocator to return to the OS any free memory
inside the process.

To access this new functionality, add a `mcctl` command to set/get CTL
properties. Example usage (and currently only supported
functionality):

    ./mcctl -h localhost:12000 set release_free_memory

Change-Id: I35c40ff9c41c0f00802ff9db9202865ec146f46c
Reviewed-on: http://review.couchbase.org/39608
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>