Sherlock serve part of data continues

[sompylasar]

PubSubHTTPEndpoint: Introduced n_min, renamed n to n_max.
 n_min is required to be able to request at least n_min entries even if there were no entries within the recent time frame.

[dkorolev]

Hi @sompylasar ,

Let me close this pull request w/o merging.

Results of my overnight sleep with this idea. We should have two flags:

1. n for the minimum number of entries to return, as in tail -n 10. The last n entries are returned.
2. cap for the maximum number of entries to return.

The behavior is:

• If only cap is set, HTTP connection terminates after cap entries have been sent. Those can be cap entries since the beginning of the stream, or since recent, or since other "data beginning" parameter.

(Note that the connection is potentially indefinite, in case the stream does yet not have cap entries to output.)

• If only n is set, cap is assumed to be equal to n. Thus, providing n results not only in at least N, but also in at most N. Use ?n=10&cap=0 to get indefinite stream with at least n entries returned.
• n overrides recent or other time-range-defining parameters. Thus, ?n=10&recent=10000 means "entries from the past 10 seconds, but do go further into the past if this is required to output at least 10 entries".

Thanks, Dima