Refactor: to explicit Java (driver) class name loading

[kares]

This is a second attempt on fixing #83 as it seems to still manifest even with locking.

Few more observations on what's going on (with #83):

• Sequel::JDBC.load_driver ends up doing an eval on the input (e.g. Java::oracle.jdbc.driver.OracleDriver)

• the eval sometimes fails with a NameError (having multiple drivers and executing threads is a factor)

• as hinted previously the problem seems to be specific to 'com.sybase.jdbc4.jdbc.SybDriver' driver (+ having another driver such as 'oracle.jdbc.driver.OracleDriver' around as well)

• JRuby might have an underlying bug with eval and setting up the Java package structure for the drivers

• at one point an ugly error came out - which indicates an underlying Java driver loading issue (although this might have been caused by the concurrent loading of different drivers - registering with DriverManager)

• the issue only happens when drivers are on the LS class-path (setting jdbc_driver_library => did not reproduce!)

• with the changes in this PR the issue did no longer reproduce even if drivers were on the LS class-path

A bit simplified reproducer (does not need to run Logstash): https://gist.github.com/kares/850767d4be4d5323b068af9585e45d4a

p.s. Previous changes (from #84) could be reverted but I am keeping them in (they do not hurt - the lock just slows down multiple jdbc plugins starting), for now.

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More investigation might be needed for an actual underlying cause (as well as the potential JRuby bug) but the changes presented here make sense regardless - skipping eval and doing a simply Class.forName is very similar to what an actual Java JDBC application would attempt to do (and should hopefully make debugging the problem easier).

[andsel]

LGTM
The red CI is due to logstash-plugins/.ci#31

Nice idea the one don't use the Sequel load_driver method and go straight to the Java way of loading.