This commit adds initial support for the jThreadsInfo packet to lldb-server. The current
implementation does not expedite inferior memory. It also does not send over all GPR, but only
the most important ones (PC, SP, FP, RA), as for some reasons writing the registers is incredibly
slow in lldb-server (I intend to investigate this further). Nevertheless, this implementation
speeds up stepping of a heavily multi-threaded inferior significantly (about 50%).
I have also added a description of the new packet to our protocol documentation (mostly taken
from Greg's earlier commit message).