crypto: add SPKAC support

Implements new class 'Certificate' within crypto object for working
with SPKAC's (signed public key & challenge) natively.
doc: crypto: document SPKAC additions

Document the SPKAC functionality that was added in commit 7bf46ba.
http_parser: expose pause/resume method for parser
http: provide backpressure for pipeline flood

If a client sends a lot more pipelined requests than we can handle, then
we need to provide backpressure so that the client knows to back off.
Do this by pausing both the stream and the parser itself when the
responses are not being read by the downstream client.

Fix GH-6214
debugger: make busy loops SIGUSR1-interruptible

Commit 30e5366b ("core: Use a uv_signal for debug listener") changed
SIGUSR1 handling from a signal handler to libuv's uv_signal_*()
functionality to fix a race condition (and possible hang) in the
signal handler.

While a good change in itself, it made it impossible to interrupt
long running scripts.  When a script is stuck in a busy loop, control
never returns to the event loop, which in turn means the signal
callback - and therefore the debugger - is never invoked.

This commit changes SIGUSR1 handling back to a normal signal handler
but one that treads _very_ carefully.
debugger: fix SIGUSR1 bootstrap race condition

Before this commit, the SIGUSR1 signal handler wasn't installed until
late in the bootstrapping process and we were prone to miss signals
sent by other processes.

This commit installs an early-boot signal handler that merely records
the fact that we received a signal.  Once the debugger infrastructure
is in place, the signal is re-raised, kickstarting the debugger.

Among other things, this means that simple/test-debugger-client is
now _much_ less likely to fail.