RadioBearerStats are not correctly connected

I am recreating this bug report from bug report created by @mrequena (https://www.nsnam.org/bugzilla/show_bug.cgi?id=3015) on Bugzilla.

Currently, there are some issues with the RadioBearerStats. A few TX/RX PDUs are missing and a few PDUs are duplicated in the traces files of PDCP and RLC.

The previous RadioBearerStatsConnector relays on the states of eNB RRC and UE RRC. But these states does not match how and when the bearers (PDCP and RLC entities) are created.

The attached patch patch-radio-bearer-stats.patch fixes how the traces are connected in RRC and creates new trace sources in eNB/UE RRC to align with the creation of the SRB1 and DRB.