Further optimize memory for Travis jobs

[jihoonson]

Fixes #6149.

I tested this PR by triggering several Travis jobs, and could see processing module succeeded 5 times in a row.

In this PR,

• Added CloseableDefaultBlockingPool and CloseableStupidPool for only unit testing to release buffers as soon as possible. Some tests can be further optimized to release buffers per parameterized run once Junit 4.13 is released which contains @BeforeParam/@AfterParam for Parameterized runner junit-team/junit4#1435. With this change, processing module testing can be run with 1500m off-heap memory.
• Changed to use different memory configuration for testing processing and others. processing requires small heap and large off-heap memory, but others are opposite. I set the default memory configuration to -Xmx1500m -XX:MaxDirectMemorySize=512m in ${DRUID_HOME}/pom.xml. processing has its own configuration, -Xmx512m -XX:MaxDirectMemorySize=1500m in ${DRUID_HOME}/processing/pom.xml. These configurations can also be set per Travis jobs using _JAVA_OPTIONS, but I think it's better to put them in pom.xml files to use those configurations whenever running tests even in local.
• Changed all unit tests to be run sequentially to use less memory. Since parallel-test spawns new processes, it's not easy to control the memory being used by each process.

[fjy]

👍