Improved message buffer handling for remote fdb

[Ozaq]

Context

This is a followup task based on the code review of remote FDB.

When sending and receiving serialized mars request we allocate a 1MB fixed size buffer. For most requests this buffer is too large but still there might be the odd request that will get rejected because the buffer is too small.

Task

We assume that the MarsRequest class contains all information required to at least provide a lower bound for the memory required to serialize a request. The MarsRequest should be extended to compute this value. The code doing the serialization should then use this information to create a buffer of the appropriate size.