Pass metadata to PDs only at runtime, not at decoder start

Pass metadata to PDs only at runtime, not at decoder start

The SRD_CONF_NUM_PROBES metadata key was removed. It wasn't actually
used for anything, since this is trivially available via the configured
(or default) probe list.

The SRD_CONF_UNITSIZE key was removed. The unit size is instead derived
from the probe list: the number of probes packed into the least amount
of space possible defines the unit size.

PD changes:
* The start() method no longer takes a 'metadata' parameter.
* Metadata now comes in only via the metadata() method, which takes
  a key and value. The only key defined so far is SRD_CONF_SAMPLERATE,
  which is exported into the module namespace.

API changes:
* srd_session_send() now takes an end_samplenum parameter, and had its
  options rearranged.
* srd_session_config_set() is now srd_session_metadata_set(). This keeps
  "config" options for a future feature to allow PDs or frontends to
  configure each other's options on the fly, up and down the stack.