539 sy 601 cesium channel rekey

[LeonLiur]

• Channel rekey
• Fixed behaviour of memFS with O_CREATE flags: no longer creates a new file when a file already exists
• File-based Int32 counter now attempts to read the given file and store as value on creation – if found a non-EOF value, it will store it as the counter

After rekey, we will lose writers to all previous files ie they will not be filled up, if they are not full – and the last written file will be not full. Considering the frequency of rekey, this is fine for now. Eventually this will be fixed by rewriting file controller, possibly during implementation of timerange deletion & tombstoning.