Fix isReadable() and isWritable() in SearchService

Summary:
Ref T12450. Minor cleanup:

• setRoles() has no callers.
• getRoles() has no callers (these two methods are leftovers from an earlier iteration of the change).
• The hasRole() logic doesn't work since nothing calls setRole().
• hasRole() has only isreadable/iswritable as callers.
• The isReadable()/isWritable() logic doesn't work since hasRole() doesn't work.

Instead, just check if there are any readable/writable hosts. Host already inherits its config from Service so this gets the same answer without any fuss.

Also add some read/write constants to make grepping this stuff a little easier.

Test Plan:

• Grepped for all removed symbols, saw only newer-generation calls in Host.
• See next diff for use of isWritable().

Reviewers: chad, 20after4

Reviewed By: 20after4

Maniphest Tasks: T12450

Differential Revision: https://secure.phabricator.com/D17571