Add: concurrent SSTs handling

It is a thing: 2 nodes joining at the same time, with 2 JOINERs and 2
DONORs cluster-wide
It can happen on operators with 2 garbd joining at the same time

Before, pt-galera-log-explainer was using SST metadata naively.
Basically if a node was DONOR and we found a "transfer completed"
message, we assumed the donor name we found is the correct one.
So for concurrent SSTs, donors were swapping names.

Now, it is handled by a map, indexed by a donor name. To know if a node
is actual donor or not, it now compare timestamps of events. It assumes
both "selected donor" and "shifting DONOR" messages should have happen
in less than 0.01 secs to avoid any conflict.

Regression tests coming in next commit with an operator logs having
concurrent SSTs. Another conflicts was sometimes breaking the test
depending on the order on which we read files, hence why it's not added
here yet

src/go/pt-galera-log-explainer/regex/regex.go

@@ -61,7 +61,7 @@ var (
 	regexSeqno         = "(?P<" + groupSeqno + ">[0-9]+)"
 	regexNodeIP        = "(?P<" + groupNodeIP + ">[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3})"
 	regexNodeIPMethod  = "(?P<" + groupMethod + ">.+)://" + regexNodeIP + ":[0-9]{1,6}"
-	regexIdx           = "(?P<" + groupIdx + ">-?[0-9]{1,2})"
+	regexIdx           = "(?P<" + groupIdx + ">-?[0-9]{1,2})(\\.-?[0-9])?"
 	regexVersion       = "(?P<" + groupVersion + ">(5|8|10|11)\\.[0-9]\\.[0-9]{1,2})"
 	regexErrorMD5      = "(?P<" + groupErrorMD5 + ">[a-z0-9]*)"
 )