While we're accumulating the 'length:' prefix, the present socket-based logic
reads 20 characters, then reads 'length' more, then discards any excess (in
case the whole 'length:data' packet ends up being less than 20 characters).
That's probably a bug: whatever characters follow that packet, however short
it may be, are probably the 'length:' prefix of the next packet. We probably
only get away with it because we probably never send packets that short.
Earlier llleap_test.cpp plugin logic still read 20 characters, then, if there
were any left after the present packet, cached them as the start of the next
packet. This is probably more correct, but complicated. Easier just to read
individual characters until we've seen 'length:', then try for exactly the
specified length over however many reads that requires.

In load testing, we have observed intermittent failures on Windows in which
LLSDNotationStreamer into std::ostringstream seems to bump into a hard limit
of 1048590 bytes. ostringstream reports that much buffered data and returns
that much -- even though, on examination, the notation-serialized stream is
incomplete at that point. It's our intention to load-test LLLeap and
LLProcess, not the local iostream implementation; we hope that this kind of
data volume is comfortably greater than actual usage. Back off the
load-testing max size a bit.

New llleap_test.cpp load testing turned up Windows issue in which plugin
process received corrupt packet, producing LLSDParseError. Add code to dump
the bad packet in that case -- but if LLSDParseError is willing to state the
offset of the problem, not ALL of the packet.
Quiet MSVC warning about little internal base class needing virtual destructor.

These tests rule out corruption as we cross buffer boundaries in OS pipes and
the LLLeap implementation itself.

It only took a few examples of trying to wrangle notation LLSD as string data
to illustrate how clumsy that is. I'd forgotten that a couple other TUT tests
already invoke Python code that depends on the llsd module. The trick is to
recognize that at least as of now, there's still an obsolete version of the
module in the viewer's own source tree. Python code is careful to try
importing llbase.llsd before indra.base.llsd, so that if/when we finally do
clear indra/lib/python from the viewer repo, we need only require that llbase
be installed on every build machine.

Migrate logic from specific test to common reader module, notably parsing the
wakeup message containing the reply-pump name.
Make test script post to Result struct to communicate success/failure to C++
TUT test, rather than just writing to log.
Make test script insensitive to key order in serialized LLSD::Map.

Instantiating LLLeap with a command to execute a particular child process sets
up machinery to speak LLSD Event API Plugin protocol with that child process.
LLLeap is an LLInstanceTracker subclass, so the code that instantiates need
not hold the pointer. LLLeap monitors child-process termination and deletes
itself when done.