Batch Maven project imports to reduce memory consumption

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As described in #824 (comment), the Maven project import mechanism could be optimized in the following ways:

9 - The maven importer imports all projects in one step - https://github.com/eclipse/eclipse.jdt.ls/blob/master/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/managers/MavenProjectImporter.java#L153. In this way, Java LS requires a lot of memory. The che project contains 299 maven projects. VS Code 0.42.0 requires 2g of heap when importing it using an empty maven local repository. I have added the following feature:

if a workspace contains more than ten projects, Java LS imports ten projects in one step and updates the workspace after importing all the projects.
This feature enables us to import the che project with 1g of heap.