Remove caching to stop memory leak

[zagor]

If you run a uModbus server that receives frequent and varying requests for an extended period of time, you will notice it uses more and more memory.

This is due to two reasons:

1. The memoize cache saves created messages, but they never expire.
   My quick fix to this is to stop using memoize.

2. Python's internal struct.pack() also has a cache, which also does not automatically expire. This is mentioned at https://bugs.python.org/issue14596 but oddly marked as fixed even though the issue remains.
   My quick fix to this is to use python's inherent list concatenation syntax instead of struct packing where possible. I replaced every use of struct.pack() where it concerned bytes, but kept struct.pack() for words (because manual endian handling is noisy).

These changes were enough to stop the memory leak in my system. I assume performance is affected but I haven't measured it.