Support logback 1.6.x in the logback toolkit layouts (#837)

* Support logback 1.6.x in the logback toolkit layouts

Logback 1.6.0 removed PatternLayout.defaultConverterMap, which
TraceIdPatternLogbackLayout and TraceIdMDCPatternLogbackLayout wrote their
conversion words to in a static initializer, so both failed with
NoSuchFieldError: defaultConverterMap (apache/skywalking#14120).

The layouts now register their conversion words in the logging context's
conversion rule registry (CoreConstants.PATTERN_RULE_REGISTRY) when they
start, updating it in place as logback does for <conversionRule>. Logback
1.2.x to 1.6.x all read class names from that registry, so the same toolkit
artifact keeps working on older logback. Logback 1.5.13 is not supported
because of an upstream regression fixed in 1.5.14 (qos-ch/logback#885).

The toolkit now compiles against logback 1.6.5, so an API removal fails the
build. LogbackVersionCompatibilityTest renders both layouts against logback
1.2.13, 1.3.16, 1.4.14, 1.5.12, 1.5.14, 1.5.38 and 1.6.5, each loaded in an
isolated class loader.

Add apm-toolkit-logback-scenario, the first plugin test of the logback
toolkit and its agent activation. It runs logback 1.2.x to 1.5.x with the
released toolkit and checks the trace ID and SkyWalking context rendered by
both layouts, including behind an AsyncAppender, through the gRPC log
reporter.

* Replace the conversion rule registry instead of modifying it

Register the SkyWalking conversion words by replacing the context's
conversion rule registry with an updated copy, under the context monitor
held only for the copy. Layouts starting concurrently, from any class
loader, keep each other's words, and logback reads a registry that is
never modified afterwards, so no lock is held while logback creates and
starts the converters. Nothing is copied once the words are registered.

On logback before 1.5.14, which also writes <conversionRule>s to this
registry, a rule logback registers at the same time as a layout starts in
another thread may be lost; this is documented.