add a call() for changing the maximum length of messages

[sebhtml]

presently, some code paths strongly assume that the maximum size of any message is 4000 bytes.

[sebhtml]

Presently, these parameters are set at compile time in:

/* a basic unit for message buffers */
typedef uint64_t MessageUnit;

#define MAXIMUM_MESSAGE_SIZE_IN_BYTES 4000

[sebhtml]

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