: Locking and simplification for TFRC TX history

This third bundle (3 more to come in this patch set) is largely self-contained does the following for the TFRC history

* implements locking
* removes unused fields (smaller footprint of structure)
* simplifies to use singly-linked list (suggestion by Arnaldo)

Little is added here, many lines are removed.

Patch #1: Removes two fields, _sent, and _rtt, which are referenced nowhere in the code.

Patch #2: Migration to singly-linked TX history -- introduction of data structures.

Patch #3: Introduction of consistent naming scheme for functions and fields located
in the dccp_tfrc_lib module: all such objects now start with `tfrc_' prefix.

Patch #4: Allocation (constructor) and de-allocation (destructor) routines for TFRC TX history.

Patch #5: Querying function to look up the send time, lock-protected against concurrent write.

Patch #6: Update CCID3 TX RTT sampling to use ktime_t
(Ian, this is one of the dependencies: upgrading to ktime_t could only be done
after introducing the TX history; ccid3_hc_tx_update_x() is resolved subsequently.