write_mongodb plugin: Fix BSON memory handling even further.

write_mongodb plugin: Fix BSON memory handling even further.

According to the mongo-c-driver 0.8.1 documentation, each call to
bson_alloc() needs to be matched by a call to bson_dealloc() and each
call to bson_init() needs to be matched by a call to bson_destroy().

Additionally, this patch adds macros for mongo-c-driver < 0.8.0 to map
the new bson_alloc() and bson_deallow() names to the old bson_create()
and bson_dispose() symbol names.

Tested with mongo-c-driver 0.8.1 and 0.7.1.

Fixes: #331, #956