Prepare Monex 2.0.0 (for eXist 5+)

[joewiz]

As proposed in #73 (comment), this PR (1) includes all changes since the 0.9.17 release, (2) "lowers" the eXist dependency to 5.0.0-RC6, since the previous dependency on 5.0.0-SNAPSHOT prevented installation of monex on 5.0.0-RC* versions of eXist, and (3) establishes the 2.x line of development for monex, which will take place in the master branch (whereas the 1.x line will take place in the monex-1.x branch).

I selected RC6 because (a) Compilation failed when targeting any lower version, as demonstrated by the following error:

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  13.153 s
[INFO] Finished at: 2019-04-13T19:51:29-04:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile (default-compile) on project monex: Compilation failure
[ERROR] /Users/joe/workspace/monex/src/main/java/org/exist/remoteconsole/RemoteConsoleEndpoint.java:[24,22] cannot find symbol
[ERROR]   symbol:   class ThreadUtils
[ERROR]   location: package org.exist.util

And (b) RC5 wasn't available in the maven repository used by monex, as demonstrated by the following build failure:

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.518 s
[INFO] Finished at: 2019-04-13T19:51:38-04:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal on project monex: Could not resolve dependencies for project org.exist-db.apps:monex:jar:2.0.0-SNAPSHOT: The following artifacts could not be resolved: org.exist-db:exist-core:jar:5.0.0-RC5, org.exist-db:exist-testkit:jar:5.0.0-RC5: Failure to find org.exist-db:exist-core:jar:5.0.0-RC5 in http://repo.evolvedbinary.com/repository/exist-db/ was cached in the local repository, resolution will not be reattempted until the update interval of exist-db has elapsed or updates are forced -> [Help 1]

[duncdrum]

see my other review: let's add semver-min and semver-max here as well

[joewiz]

@duncdrum I've now changed xar-assembly.xml here too but am not entirely convinced of the need to set semver-max on monex 2.x until we know this version actually has an upward incompatibility.

[duncdrum]

Setting min.version should indeed be sufficient here