Finally tracked down the racy condition that would hose up our debug

sessions: When continue packet has been sent and an interrupt packet was
quickly sent, it would get read at the same time:

$c#00\x03

There was an error where the packet end index was always being computed 
incorrectly by debugserver, but it wouldn't matter if there weren't extra
bytes on the end (the hex \x03 interrupt byte in this case). The first
'$' last 3 bytes of the data in the packet buffer were being trimmed
(trying to trim the '#' + checksum (#XX)) which made:

c#

And this would then be passed to the handle routine for the 'c' packet which
would see an extra character at the end and assume it was going to be in the
form c[addr] where "[addr]" was a hex address to resume at and this would
result in a malformed packet response. This is now fixed and everything works
great.

Another issue was issuing async packets correctly by doing correct handshakes
between the thread that wants to send the async packet, and the thread that
is tracking the current run.

Added a write lock to the communication class as well to make sure you never
get two threads trying to write data at the same time. This wasn't happening,
but it is a good idea to make sure it doesn't.

llvm-svn: 124369