Support polling with no delay

[skliper]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the CF README.md file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
Polling has a timer that doesn't support zero delay, limiting performance:

CF/fsw/inc/cf_tbldefs.h

Lines 36 to 40 in 930b0e5

	uint32 interval_sec; /**<
	* \brief number of seconds to wait before trying a new directory.
	*
	* Must be >0 or slot is inactive.
	*/

CF/fsw/src/cf_cfdp.c

Lines 1513 to 1515 in 930b0e5

	if (pd->enabled && pd->interval_sec)
	{
	/* only handle polling for polldirs configured with a non-zero interval */

CF/unit-test/cf_cfdp_tests.c

Lines 1144 to 1148 in 930b0e5

	/* nominal call, w/engine enabled, polldir enabled but interval_sec == 0 */
	CF_AppData.engine.enabled = 1;
	pdcfg->enabled = 1;
	UtAssert_VOIDCALL(CF_CFDP_ProcessPollingDirectories(c));
	UtAssert_UINT32_EQ(CF_AppData.hk.channel_hk[UT_CFDP_CHANNEL].poll_counter, 0);

Describe the solution you'd like
There's already a channel enable, no need to use interval to disable the channel. Update to support 0 delay to maximize throughput when using the polling directory.

Describe alternatives you've considered
None

Additional context
I can't say I really understand why 0 timeout wasn't supported to begin with. If anyone knows I'd be interested to hear it. In rate constrained situations unnecessary delays on file transfers are a big impact. Sending small files on a fast link could waste significant bandwidth by waiting a second between polling.

Requester Info
Jacob Hageman - NASA/GSFC