[RocksJava] LogLevel support in Options

It's now possible to set a LogLevel in Options and
DBOptions to control LOG verbosity.
[RocksJava] Incorporated review comments D28947
[RocksJava] Bump version to 3.8 in rocksjni.pom
[RocksJava] MVN Build reads version from version.h
Merge pull request #399 from fyrz/RocksJava-Version-to-3.8

[RocksJava] Bump version to 3.8.0 in rocksjni.pom
Merge pull request #396 from fyrz/RocksJava-LogLevel

[RocksJava] LogLevel support in Options
Improve the comment in InfoLogLevelTest.java

Summary:
Improve the comment in InfoLogLevelTest.java

Test Plan:
make rocksdbjava
[RocksJava] Sigsegv fix for MergerOperatorByName