Investigate caching String to Bytes conversions and visa versa

[keith-turner]

Since String and Bytes are both immutable, it may be worthwhile to cache conversion from one to the other. The following are some thoughts on possible implementations.

• Per thread cache w/o synchronization
• Global cache with synchronization
• Hybrid per thread/global cache
• Single threaded short lived per operation/per transaction caches.

[ctubbsii]

Accumulo's client code has suffered from excessive caching of objects globally/statically in the JVM, all outside the user's control. If a caching feature is introduced, the user should have control over the size and lifecycle of the cache, to learn from Accumulo's lessons.

Of the options, I think the short-lived cache is most appealing. But, if it's user-provided, they could selectively re-use the same cache for many operations.

[keith-turner]

In #804 I did some very narrow caching. This caching is only at a per transaction level and in some cases only per a transactions operation.