camel-store-summary.h readability cleanups.
CamelStoreSummary: Remove unused "store_info_chunks" member.

This reduces the size of CamelStoreSummary and therefore requires a
libcamel soname bump.
Remove CAMEL_STORE_SUMMARY_REF_LOCK.

Use atomic integer operations to ref/unref a CamelStoreInfo struct.
Add camel_store_summary_info_unref().

Replaces camel_store_summary_info_free(), although for the moment we'll
retain an alias for backward-compatibility.

Calling a function "free" when it actually only decrements a reference
count is very misleading.
Remove CAMEL_STORE_SUMMARY_IO_LOCK.

Keep this internal to CamelStoreSummary.
Remove unused camel_store_summary_header_load().
Remove unused camel_store_summary_info_new_from_path().
Remove unused camel_store_summary_remove_index().
Remove unused camel_store_summary_clear().

Was only called from CamelStoreSummary's finalize().
Convert camel_store_info_path() macro to a function.
Convert camel_store_info_name() macro to a function.
Avoid camel_store_info_string().

It's not like calling the function is any more thread-safe than just
accessing the CamelStoreInfo struct member directly.

Avoid the function so we can get rid of it.
Remove camel_store_info_string().

No longer used, and the class method was pointless.
Remove unused CAMEL_STORE_INFO_NAME.

CAMEL_STORE_INFO_NAME is just a variation of the path string,
and makes no sense to use with camel_store_info_set_string().
Avoid camel_store_summary_index().

camel_store_summary_index() is only called when looping over summary
items from 0 to camel_store_summary_count().  But this pattern is not
thread-safe, because another thread could add or remove summary items
while the loop is in progress.

Use camel_store_summary_array() instead to avoid these issues.