Cache Maven Local Repository

Now that we're on the Travis CI container based infrastructure we can
use caching for the local Maven repository [1] in order to speed up the
build times and reduce the load on Maven Central.

 [1] https://docs.travis-ci.com/user/caching/

* ISSUE-404 EXPLAIN should process statements with params placeholders.

* ISSUE-404 Fixed CommandRemote to support EXPLAIN with params placeholder.

* ISSUE-404 Fixed CommandRemote to support EXPLAIN with params placeholder.

* ISSUE-404 Support EXPLAIN with params placeholder WIP.

* ISSUE-404 Support EXPLAIN with params placeholder WIP.

* ISSUE-404 Support EXPLAIN with params placeholder WIP.

* ISSUE-404 Support EXPLAIN with params placeholder WIP.

* ISSUE-404 Muted tests.

* ISSUE-404 Fixed review notes.

* ISSUE-404 Un-muted tests.

no reason to be parsimonious here, can only help

CI still failing, maybe we have a memory leak somewhere in the tests?

so performance doesn't bottleneck on the synchronization in the FileBase
default implementations

otherwise client code will end up calling the version in FileBase, which
has unnecessary synchronisation

so we don't get warnings about the overriding methods also needing to be
synchronized, which is not the case

we're not interesting in exceptions in closing in these tests

since we require Java7, this is no longer necessary

to give us a thread dump when they get stuck

About #385 

Fix multi-threaded mode insert exception "Unique index or primary key violation", test case by Anatolii K

so we get nicer error messages when things fail

when run in memory mode, the initial connection used a memory DB,while
the second connection used a disk DB

doing DROP ALL OBJECTS

leads to an NPE when the session closes