Requests to dashboard and user list block when repositories are upgrading

When upgrading FishEye from 2.3.3 to 2.10.3, FishEye become unresponsive when accessing dashboard ( / ) or people ( /users ) pages.

There are only 3 initial indexing threads in the server.
When 3 repo started, it will perform indexing and the server is still working.
However, if more than 3 repo is started, the server hung.

Found the following stacktrace through DynaTrace when the server hung:

RepositoryHandle.acquireEngine()
  CommiterUserMappingManagerImpl.getImplicitCommitersForUserInRep(String, User)

Could be related to https://jira.atlassian.com/browse/FE-3932