Use getPDPConfig() instead of individual PDP config getters

[rvagg]

WarmStorageService.getMaxProvingPeriod() and WarmStorageService.getChallengeWindow() should call getPDPConfig() on the view contract instead of the individual getter functions.

• getPDPConfig() is defined by IPDPProvingSchedule interface - the canonical way to retrieve these values
• Individual getters (getMaxProvingPeriod(), challengeWindow(), getChallengesPerProof()) are redundant and may be removed from the view contract (Deprecate challengeWindow() function filecoin-services#336)
• Single call is more efficient than two separate calls

In src/warm-storage/service.ts we have getMaxProvingPeriod() and getChallengeWindow(), I think we should just remove those and add a getPDPConfig(). Then they are used in src/storage/context.ts:1378 in a Promise.all and that can be replaced with a single getPDPConfig() call.

[akronim26]

@rvagg, can I work on this?