[obexd] Chunk PBAP pull requests

Do not supply all PBAP content to a client's pull request in one
response, but request data from obexd-calldata-provider in chunks and
pass them along to the client one by one. This brings down the latency
between request from a client and the first response packet, even if
the total time to serve the request may slightly increase due to
increased roundtrips.
[obexd] Get only metadata for cache

For filling up the PBAP core implementation cache, vCard data is not
needed. Request only metadata from obexd-calldata-provider. This will
cut the time needed to provide the response.
[obexd] Code rearrangement

Moved if/else blocks closer to where computed values are used.
Merge pull request #22 from hmallat/PBAP-request-optimization

Pbap request optimization