ivs: tune malloc trim threshold

When processing a batch of messages in a single tick of the event loop, we
allocate a 64 KB block per message and then free them all at once after writing
to the socket. Looking at the strace, I noticed we were doing a brk() for every
message allocation. When freeing the messages we did another brk() which
released all the memory. The allocator has a limit on the amount of free memory
it will keep in the process before it starts releasing it back to the OS. Our
malloc churn is larger than the 128 KB default, so this change adjusts the trim
threshold to 2 MB.

This improves performance in my echo benchmark by 150%.
ivs: add malloc include
Merge into master from pull request #143:
ivs: tune malloc trim threshold (https://github.com/floodlight/ivs/pull/143)