Provide memory & gc options in bookkeeper script

[sijie]

Descriptions of the changes in this PR:

It is convenient to provide MEM_OPTS and GC_OPTS in bookkeeper script and bkenv.sh to allow overriding jvm settings through.

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[eolivelli]

Looks good.

As you are working on this can you take a look at adding JAVA_JDK_OPT and add jdk internal module 'opens' to netty and orur codebase in order to hide 'illegal reflective access' warnings while running on jdk10?

I can do in a follow up change if you do not have time

[sijie]

dk internal module 'opens' to netty and orur codebase in order to hide 'illegal reflective access' warnings while running on jdk10?

Please do this as a separate issue. I don't use jdk10, which I can't verify any changes.

[eolivelli]

+1 lgtm

[sijie]

retest this please

[sijie]

retest this please