Improve Get/Put performance with optional mempooling (#36)

* avoid unnecessary use of encoder/decoder to decrease memory allocations

* add an optional configurable mempool to avoid extra allocs

* add doc.go with examples

doc.go

@@ -0,0 +1,13 @@
+// Package bitcask implements a high-performance key-value store based on a
+// WAL and LSM.
+//
+// By default, the client assumes a default configuration regarding maximum key size,
+// maximum value size, maximum datafile size, and memory pools to avoid allocations.
+// Refer to Constants section to know default values.
+//
+// For extra performance, configure the memory pool option properly. This option
+// requires to specify the maximum number of concurrent use of the package. Failing to
+// set a high-enough value would impact latency and throughput. Likewise, overestimating
+// would yield in an unnecessary big memory footprint.
+// The default configuration doesn't use a memory pool.
+package bitcask