Add waitfor CLI command

[prkrtg]

Adds the ability to block until a keyword satisfies a condition, modelled on
KTL's waitFor.
https://www2.keck.hawaii.edu/software/KTLPython/examples.html#using-waitfor

Because libby daemons don't broadcast keyword changes, wait_for polls the
keyword over RPC rather than waiting on a monitor; --poll sets the interval
(default 0.1s)

Example usage:

$ libby waitfor '$hsfei.pickoff.ismoving == false' --timeout 30
hsfei.pickoff.ismoving = False (satisfied after 4.2s)

$ libby waitfor '$hsfei.pickoff.positionvalue > 15' --timeout 30 --json
{
  "ok": true,
  "expression": "$hsfei.pickoff.positionvalue > 15",
  "satisfied": true,
  "qualified": "hsfei.pickoff.positionvalue",
  "value": 79.0,
  "elapsed_s": 6.104,
  "polls": 62
}

via Python

if client.wait_for("$hsfei.pickoff.positionvalue > 15", timeout=5):
    print("pickoff exceeded 15 within 5 seconds")

client.wait_for("$ismoving == false", 30, service="hsfei.pickoff")

[mikelangmayr]

Looks good!