Add jThreadsInfo support to lldb-server

Summary:
This commit adds initial support for the jThreadsInfo packet to lldb-server. The current
implementation does not expedite inferior memory.  I have also added a description of the new
packet to our protocol documentation (mostly taken from Greg's earlier commit message).

Reviewers: clayborg, ovyalov, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11187

llvm-svn: 242402